Most of these flash suppressors were welded on at the factory, that is why they did not bother to tighten the castle nuts properly. The weld was supposed to be doing the actual work. But, it is a shallow spot weld, and the surfaces were not properly prepped, so most of them are very weak.
With mine, the recoil of about 300 rounds broke the welds, allowing the flash suppressor to wobble. I bet something similar has happened to yours, with the weld only partly broken. Thus, you tighten the screw, removing the slop, but then the remaining weld metal yields a bit further, generating more slop.
Have you completely removed the unit, cleaned out any and all grit and such, and cleaned away any ligaments of metal with a file before checking to see that the unit slides on neatly all the way before bottoming out on the splines? If you do this I think it should tighten up once and for all.
