Entry delay / alarm only starts sometimes when entering front door

My entry delay, when entering through the front door, only works about 1/3rd of the time. When I enter my home the doorbell sound chimes, but the entry delay doesn't start. Basically I'm in the house and nothing happens. The siren won't sound, because it doesn't register for some reason. 1/3rd of the time or so, it does work, and I have to enter the code, as it should always be. This is very troubling, because it makes the entire system worthless. I have checked all the help forums - everything is set-up. It is a main door contact sensor, set to main door, with a 60 second entry delay. It works periodically. When it does not work (the entry delay doesn't start), it still makes the chime sound. So the contact is working, but it's not setting off when I come home. I noticed this specifically is a problem when we're gone for at least an hour or so. If I try to "test" it out, it works every time, just opening the door.