Sounds like the gun wasn't locked up in the first place. Rather, the hand (rather than the bolt) is indexing the cylinder and it is falling away when the hammer is lowered.
Get a light on the bolt to see if it is, in fact, locking up.
I'm sure there are different opinions on this, but many feel it is satisfactory to have inertia carry the cylinder to full lock when the timing of the secondary ratchet falls short.
So, Does it lock up when run at normal speed??? Does the failure to lock up only occur during very slow operation? Once again, are you certain that it locks up at all?