Do you see any movement at all on the locks? Like it is trying, but just doesn't have the power to complete it? Most older Fieros require multiple presses of the button to make the locks work.
Either way...it is either a weak solenoid, or a physical problem with the rods...not an electrical issue. First, I would pull the door panel, and clean/regrease the rods to make sure they are working smoothly. Also, verify the rod hasn't fallen off the solenoid. If that doesn't fix it, then replace the solenoids.