I had this problem. The KFC 150 would sometime pass the test on ground and sometimes fail. The yoke switch would work, intermittently. The A/P would not trim. Took the bird to Executive Autopilots at KSAC. Dave found two issues, one was a bad solder on the switch post. He re-tested and could could hear the clutch engage and hear the motor drive but it wouldn't move the controls. On bench testing the servo he found the clutch wasn't fully engaging, he cleaned and re-aligned the engage assembly and voila! problem solved.
Highly recommend Executive Autopilots, class acts from start to finish