It could be that there is not enough tension on the return spring, the spring tension is easily adjustable by using a different screw position to secure the spring to the bush/playfield. Ideally the spring tension should be enough to return the flipper but not too much, as the flipper power is weakened if the mechanism is fighting the spring.
I saw that you said it works ok manually but if the playfield was up when you tested it, that can make it behave differently from when the playfield is down.