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Please see the official X‑Plane Support page for help.
Make sure to remove all plugins or custom scenery, then if it remains, go ahead and file a bug report. Be as specific and detailed as possible, and include proof that a characteristic of the simulator is wrong if you believe it to be so (your real world experience should help here). A good format is to list the exact steps you take up to the problem point, then to list the expected behavior vs the actual sim behavior.
I'm an user, not support, but I thought I will share. I initially had problem with autopilot in several planes (e.g. Cirrus SF50) but this has changed with recent updates.
I have never had problems with 737 (all knobs work well) or 747 (v/s works in -400 and -100), but I noticed that sometimes I need to move slightly right or left in the cockpit to use knobs.
What is more autopilot will make mistakes if simulation is not fast enough (e.g. if I speed up simulation autopilot will have troubles keeping plane leveled). Is it possible that your simulation is running with lower than mentioned in manual ~20FPS?
This site is no longer being actively maintained.