Sounds like a sticky actuator cable. Sometimes you can lube them and sometimes you have to change them. Test by 1. engine off. 2. disconnect at transmission lever on transmission.3. lever should move by hand with medium effort. If it does, your cable is bad.(rusted or kinked or squashed ) . if it doesn't, then there is a problem in the shift index system on the side of the transmission. I wouldn't tackle that without a workshop manual.