diff --git a/lib/galaxy/workflow/modules.py b/lib/galaxy/workflow/modules.py
index 87fc5894eb5..fd6571660f7 100644
--- a/lib/galaxy/workflow/modules.py
+++ b/lib/galaxy/workflow/modules.py
@@ -508,7 +508,7 @@ class PauseModule( WorkflowModule ):
         return state
 
     def execute( self, trans, progress, invocation, step ):
-        progress.mark_step_outputs_delayed( step )
+        progress.mark_step_outputs_delayed( step, why="executing pause step" )
         return None
 
     def recover_mapping( self, step, step_invocations, progress ):
@@ -522,7 +522,8 @@ class PauseModule( WorkflowModule ):
                 return
             elif action is False:
                 raise CancelWorkflowEvaluation()
-        raise DelayedWorkflowEvaluation()
+        delayed_why = "workflow paused at this step waiting for review"
+        raise DelayedWorkflowEvaluation(why=delayed_why)
 
     def do_invocation_step_action( self, step, action ):
         """ Update or set the workflow invocation state action - generic
@@ -834,7 +835,8 @@ class ToolModule( WorkflowModule ):
                 workflow_invocation_uuid=invocation.uuid.hex
             )
         except ToolInputsNotReadyException:
-            raise DelayedWorkflowEvaluation()
+            delayed_why = "tool [%s] inputs are not ready, this special tool requires inputs to be ready" % tool.id
+            raise DelayedWorkflowEvaluation(why=delayed_why)
 
         if collection_info:
             step_outputs = dict( execution_tracker.implicit_collections )
@@ -1020,7 +1022,9 @@ def load_module_sections( trans ):
 
 
 class DelayedWorkflowEvaluation(Exception):
-    pass
+
+    def __init__(self, why=None):
+        self.why = why
 
 
 class CancelWorkflowEvaluation(Exception):
diff --git a/lib/galaxy/workflow/run.py b/lib/galaxy/workflow/run.py
index 190c2d9d67e..0619f19d6df 100644
--- a/lib/galaxy/workflow/run.py
+++ b/lib/galaxy/workflow/run.py
@@ -147,6 +147,7 @@ class WorkflowInvoker( object ):
         workflow_invocation = self.workflow_invocation
         remaining_steps = self.progress.remaining_steps()
         delayed_steps = False
+        more_info = ""
         for step in remaining_steps:
             step_delayed = False
             step_timer = ExecutionTimer()
@@ -165,8 +166,9 @@ class WorkflowInvoker( object ):
                     # https://github.com/galaxyproject/galaxy/issues/2259
                     if job:
                         workflow_invocation_step.job_id = job.id
-            except modules.DelayedWorkflowEvaluation:
+            except modules.DelayedWorkflowEvaluation as de:
                 step_delayed = delayed_steps = True
+                more_info = "(%s)" % de.why
                 self.progress.mark_step_outputs_delayed( step )
             except Exception:
                 log.exception(
@@ -177,7 +179,8 @@ class WorkflowInvoker( object ):
                 raise
 
             step_verb = "invoked" if not step_delayed else "delayed"
-            log.debug("Workflow step %s of invocation %s %s %s" % (step.id, workflow_invocation.id, step_verb, step_timer))
+            status = step_verb + (" %s" % more_info if more_info else "")
+            log.debug("Workflow step %s of invocation %s %s %s" % (step.id, workflow_invocation.id, status, step_timer))
 
         if delayed_steps:
             state = model.WorkflowInvocation.states.READY
@@ -207,14 +210,16 @@ class WorkflowInvoker( object ):
 
         # No steps created yet - have to delay evaluation.
         if not step_invocations:
-            raise modules.DelayedWorkflowEvaluation()
+            delayed_why = "depends on step [%s] but that step has not been invoked yet" % output_id
+            raise modules.DelayedWorkflowEvaluation(why=delayed_why)
 
         for step_invocation in step_invocations:
             job = step_invocation.job
             if job:
                 # At least one job in incomplete.
                 if not job.finished:
-                    raise modules.DelayedWorkflowEvaluation()
+                    delayed_why = "depends on step [%s] but one or more jobs created from that step have not finished yet" % output_id
+                    raise modules.DelayedWorkflowEvaluation(why=delayed_why)
 
                 if job.state != job.states.OK:
                     raise modules.CancelWorkflowEvaluation()
@@ -292,7 +297,8 @@ class WorkflowProgress( object ):
             raise Exception(message)
         step_outputs = self.outputs[ output_step_id ]
         if step_outputs is STEP_OUTPUT_DELAYED:
-            raise modules.DelayedWorkflowEvaluation()
+            delayed_why = "dependent step [%s] delayed, so this step must be delayed" % output_step_id
+            raise modules.DelayedWorkflowEvaluation(why=delayed_why)
         output_name = connection.output_name
         try:
             replacement = step_outputs[ output_name ]
@@ -313,7 +319,8 @@ class WorkflowProgress( object ):
                     # TODO: consider distinguish between cancelled and failed?
                     raise modules.CancelWorkflowEvaluation()
 
-                raise modules.DelayedWorkflowEvaluation()
+                delayed_why = "dependent collection [%s] not yet populated with datasets" % replacement.id
+                raise modules.DelayedWorkflowEvaluation(why=delayed_why)
         return replacement
 
     def get_replacement_workflow_output( self, workflow_output ):
@@ -338,7 +345,10 @@ class WorkflowProgress( object ):
     def set_step_outputs(self, step, outputs):
         self.outputs[ step.id ] = outputs
 
-    def mark_step_outputs_delayed(self, step):
+    def mark_step_outputs_delayed(self, step, why=None):
+        if why:
+            message = "Marking step %s outputs delayed (%s)" % (step.id, why)
+            log.debug(message)
         self.outputs[ step.id ] = STEP_OUTPUT_DELAYED
 
     def _subworkflow_invocation(self, step):
@@ -395,8 +405,8 @@ class WorkflowProgress( object ):
     def _recover_mapping( self, step, step_invocations ):
         try:
             step.module.recover_mapping( step, step_invocations, self )
-        except modules.DelayedWorkflowEvaluation:
-            self.mark_step_outputs_delayed( step )
+        except modules.DelayedWorkflowEvaluation as de:
+            self.mark_step_outputs_delayed( step, de.why )
 
 
 __all__ = ( 'invoke', 'WorkflowRunConfig' )
diff --git a/test/api/test_workflows.py b/test/api/test_workflows.py
index 38261d06812..2c42a528596 100644
--- a/test/api/test_workflows.py
+++ b/test/api/test_workflows.py
@@ -869,8 +869,6 @@ test_data:
     @skip_without_tool( "cat1" )
     @skip_without_tool( "collection_paired_test" )
     def test_workflow_run_zip_collections( self ):
-        # A more advanced output collection workflow, testing regression of
-        # https://github.com/galaxyproject/galaxy/issues/776
         history_id = self.dataset_populator.new_history()
         workflow_id = self._upload_yaml_workflow("""
 class: GalaxyWorkflow