Not too familiar with car alarms. Its an OLD viper alarm that locks and unlocks the doors when you arm/disarm the alarm. The power locks still can lock the door from the inside and the alarm still unlocks the doors when you disarm the alarm, but it doesn’t lock the doors when you arm the car. Any ideas?
Check the relays. There are two relays for doing this purpose, one for unlocking and one for locking.
Locate the relays (black boxes usually) under the steering cloumn or thereabouts and hold them as you arm/disarm the car. Once you find the correct pair, see if the both of them click when arming or disarming. They may also have inline fuses as well under the dash.
Try changing the fuses and then switching the two relays to determine the source of the problem.