Blue Iris shows Occupied for alert. Why?

I’m using DeepStack with Blue Iris. When motion occurs on the camera, quite a few times I am seeing “DeepStack: Alert cancelled [occupied]”. I see a person in the camera, but no alert is generated. I do see a rather large process time on the alert, 2783ms in one instance, and as high as 13000ms. I don’t know what to do to get this ms time down, and how to fix the occupied alert.
I have DeepStack set for Real Time Images : 10, and Analyze one each : 250ms.