Take the one that does not work, get in the car, close door.

Put key in ignition, do not turn key.
Press unlock or lock twice.
Within ten seconds, turn key to "ON" (not accessory, but ON, just before "START".
Turn key off, remove key.
Get out of car, close door.
Try the lock button, it should now work.

If if still won't work, inspect the interior and make sure the battery contact tab did not break off.