Autopilot not Working.

My autopilot altitude hold will not engage on my default aircraft. I have used the mouse and the Z-button to engage it. When I engage the autopilot hold switch the the planes do not travel to their assigned altitudes at the selected vertical speeds. I still have significant elevator control but not as much when the autopilot is off. When I disengage the autopilot my controls return to normal. Has anyone encountered this problem and solved it? Help in the positive conclusion to this problem will be thanked.