Thank you, I’ve a suspicion the [EE] pin is caught on the underside of the adaptor.
According to some sources:- To mount a lens that does not have the Al facility: [ie my adaptor] Lock the lever up out of the way by pushing the coupling lever release and manually raising the lever.
Any idea how can this be done on a F-301 / N2000 body - I cannot identify a coupling lever release and the lever does not lock when moved a fraction upwards??????
I think the coupling lever is under the prism on some older bodies and can be flipped up for use on non AI lens,
A while back you put a feeler gauge between the adapter and camera body mount and said all was clear. Put a feeler as thick as possible over the lens release and EE pins then try to turn the adapter again. Its also possible a screw on the adapter mount came loose and is catching in one of the body mount screw holes. If this is the case disassembling the adapter from the front will be necessary to access the loose and tighten it enough to get the adapter to turn.
Congratulations!
Inspect the mounts for burs, bends, nicks that the EE pin or lock pin can stick in.
I would go as far as to remove the mount ring from both the camera body and adapter to check them for flatness. Replace if any unevenness is found.