Make sure fluid is at right level because most probably it is caused by improper fluid level or thickness of fluid. Also check your clutch linkage or shift cable and adjust them if necessary. One more problem that causes gear shift problem is vehicle’s computer system that controls the automatic transmission (as you see at buyusedengine.com). You need to reset it. Resetting takes most likely 30 minutes but if even after resetting, it doesn’t work, time to go for a mechanic.