I have had this problem with couple of door sensors (but not all in same time). What I found is that the rf signal reaching these sensor were borderline weak... just enough but not quite. Most of the time they work, 1 out of 5 times they don't.... when they don't.. they stay off for a while. Interestingly, I never had problems arming the system with these problem sensors. I had to rotate or reposition (it was on metal... rebar I would guess) in my house. I have 10 door/windows sensors deployed and will eventually have a total of 17 seventeen.