Motion detector with a mind of its own

The motion detector on our camera seems to have a mind of its own. For a time, it wouldn't pick up anything. I checked all the settings and nothing had changed. Now, it is going off randomly. We used to check to see if someone was actually there (usually not), now we just ignore it. Again, I have checked all the settings I can see, and nothing is odd or not normal. The camera works, the doorbell itself works, sounds are on, just the motion detector is the problem. Any ideas what's going on and how to remedy it?