If the motor just spins and does not move the mirror head then the clutch is loose. Move the mirror glass down so there is a gap at the top. Look down behind the mirror and with a long flat blade screw driver, start loosening the plastic fingers that hold the mirror on. Be careful not to break them. After you get the mirror off there will be a black plastic cover in the lower right corner, 3 screws hold it on. Remove this cover. Be careful not to move the clutch and gears under the cover. There will be a 7/16 (I think) nut in there. Tighten this nut a little bit. This will tighten up the clutch, careful not to get it to tight. Hopefully all the gears stayed in the housing and you can put the cover back on and test the movement of the head.
I should have asked if these are the new style mirrors that could be heated. If they are then the above should work, if not the call power vision for help. 800-337-2557
Not affiliated with Power vision just had to do this fix to mine.