There is adjustment that can be made to the gear selector cable. First, select P, then down on the left hand side of the transmission you should find the cable/clamping/adjustment bracket. This holds the cable in a locknut type situation. Loosen the locknut (to free up the cable), while holding the outer clamping sleeve stationary to prevent bending or damaging the cable.Push the manual valve lever forward, towards the engine and retighten the clamping nut. It's all a bit like the cable adjustment that you might find on the brakes on a bicycle! This might require a little trial and error to get it just right, but usually solves the sort of problem that you're having. Hope it helps.