False alarms on 3 entry sensors

Hi - I've had three entry sensors cause false alarms.  All are placed correctly || and in same plane less than 2 in apart.
All show the blue light correctly when opened/closed.  
All have been installed and working fine for weeks.
Only way I can make the system recover is to repeatedly open/close that sensor until it functions again(something I am NOT doing at 3am after being woke. From a sound sleep for a false alarm.)

Suggestions?