I've installed the Alert Manager App on my search head and the TA on my search head and indexers. Fired alerts were coming into the Incident Posture dashboard just fine. I could see the alerts and update/assign them. Then it just seemed to stop working. I can't see any alerts in the lower portion of the Incident Posture dashboard now. I can only see the upper portion of the Dashboard (where trending information and "recent incident" drop-downs are displayed)
The alerts are there (i.e. I can see them when I go to Reports->Incident Export. However, there seems to be an issue with seeing them via the Incident Posture dashboard itself. When I edit the panels, the "Alert Results" portion of the dashboard reads "Search is waiting for input..."
I had the EXACT same problem. The app worked fine for about a month and then one day incidents just stopped appearing in the Incident Posture dashboard. Bizarre. I too resolved the issue by reinstalling the add-on. I also reinstalled the app.