[Helix_FR]

Swing the cylinder out and take a look at the thumb piece bolt while pushing forward. Is it flush with the breech face? If not, the locking bolt (thumpiece) needs to be refit or replaced.
If it is flush, then the extractor rod was undercut or the yoke has a excessive amount of end shake.
End shake:
Check the yoke or crane for a forward and back motion in the frame. The yoke should remain flush to the frame any excessive movement must be fixed. Its not terribly hard to do. Let me know the results. PM would work better for this. This is not a home job, you can really screw it up if its done wrong.

Undercut extractor bolt:
Some trimming of the thumbscrew can be done to alleviate the problem but too much can cause the cylinder to not lock fully. I often times do this only as a last resort and only if its just a few strokes of a file. Otherwise the extractor bolt needs to be replaced.