Originally posted by Rocnal ...something is getting caught on the pin that is depressed when the lens unlock button is depressed...
I have several lenses that have screw holes on the flange face that tend to get caught up in the release pin and/or the AF drive. It is a good practice to retract the AF drive pawl and depress the release pin when mounting/removing your M42 lenses. Oh, and one other thing...screw the adapter to the lens and mount the combo to the camera.
Steve
P.S. Some of the Super Takumars, FSU, and German M42 lenses have very small diameter lens barrels. These may not properly clear the AF drive (not wide enough to depress it) and can actually damage it. Again, always flip the AF switch to MF.