Two last thing (although I doubt either will be the case):

1) Check that the shutter is touching the helical mount (i.e. not sitting out a mm or two). There isn't a second retaining ring on the thread? You haven't cross-threaded the ring? It seems that you are very close with this lens (given the closer to infinity you focus, the less movement is required).

2) Have you got the right hole size for your shutter? Again, if the hole is too small, the lens cound be sitting off the mount.

Both seem too trivial to be right, but it's hard to tell without seeing the actual item.

Lachlan.