I brought a central door locking kit. I installed and everything works fine. except one minor problem. The door on the rear passenger side. When the engine is off, the door doesn't lock it but I can hear the motor trying but can't. But when the engine runs, it works perfectly fine. I need an opinion on the problem... which solution might be.
1. There isn't enough electric current running
2. The actuator motor is bad cause it too weak

Give it a good inspection, check for binding of the linkage (something too tight) Check and re-seat electrical connections and measure voltage reaching actuator.