See Troubleshooting in the KM Wiki.
As soon as it fails, examine the KM Engine log to confirm the macro ran, or did not run.
From the KM Editor, menu Help > Open Logs folder.
You can also run this macro:
MACRO: Display Last KM Macro Error Ver 4 [Pub]
The macro also has an option to get the last N entries in the KM log.
Add an Acton to your macro that displays a notification as the first Action.