Before buying a new derailleur on the off chance that yours is shot or has a weak spring, test that theory.

1- shift to any gear. Press the lower body in to shift one position lower and let go. If the rd properly shifts back, the problem is not the RD, though the spring may not be strong enough to overcome cable friction.

2- or if you buy into the weak spring theory (I don't), you can insert a hex key into the mounting bolt to make a gallows over the RD. Loop a rubber band around the lower end of the pantograph, or some other spot near the lower body, and hang it from the gallows to add some return spring force. If this solves the problem, then the spring is not strong enough to overcome cable friction, but you still don't know if it's a weak spring, or excess friction, since the net effect is the same. In this case, my money is still with cable friction.