check tha actuater is working first by pressing the remote and see if the button moves, when the spring has gone the mechanism unlocks with the actuator but bounces back to the lock position, hence the need to lift the button manually to open the door

ive had this on three of my 5 doors in the last 12 months, its a small round spring in the lock mech that hold it either locked or unlocked that has worn thin and snaped . you can buy on e bay individualy Item number: 140020512607 findly to fit but not hard regards

i keep mine in the compartment under the passenger seat, have you looked there?, just a thought