There is probably a problem with the shift selector at the transmission or the shifter cable needs adjusting. Check the gear selector at the tranmission first and make sure it is not loose or binding. Then see where the cable attaches to it and see if it seems to move at all. If it does not it may be bound up between the shifter and the gear selctor. If the gear selctor seems to be okay. Then you will have to take the trim panels off the shifter and inspect it for loose or binidng components. You may also have to dis-connect the cable at one end to ensure that it is moving freely and moving the gear selctor or can be moved by the shifter.