Have you tried double pressing the Partial button? I have to do that if a door with a sensor on it is open or ajar or motion is detected in the case of setting the alarm to On. Since I'm still running Simplisafe parallel to Iris, part of SS arming (Home) is to have the door open, which can cause confusion if in a hurry. I have my wife double press the arming buttons to avoid no arming at all. Than once the guilty culprit, motion sensor for example, than everything is fine.
Pressing it twice is the bypass function. Perhaps a sensor is ajar or motion is detected, depedning on how you set things up.
The sensors could be slightly move during the course of repeated opening and closing the door. The connection could be lost. You can try to buy the door hinge and might find better success in the connection. Battery level in the door sensor could also be a problem.