1999 Honda Accord Doors won't always lock/unlock with remote

1999 Honda Accord Automatic

In 2003, I had an aftermarket alarm system, automatic starter, and keyless entry installed in my car. The remote control activates the alarm system by pushing the lock button, which is also supposed to lock all of the car doors, while the alarm is deactivated with the unlock button and that unlocks all of the doors. Within the past 6 months to a year, I've had to start manually unlocking/locking some of the doors because they don't always work with the remote. Even if I try using the automatic lock/unlock feature that was factory installed in the car, they won't lock/unlock as you wish. This has been a real headache. Do you have any suggestions? Thank you!