The standard J-Arm has been known to have some tolerance issues when interfacing with various mounts. The horn was developed with somewhat loose tolerance in order fit a variety of mounts made by different manufacturers. Much like the M1913 Picatinny Rail Spec, manufacturers have a hard time following dimensions, so there are various tolerances found in these mounts. J-Arms are also made by a few different manufactures, so there are tolerance issues there too. In some cases, these tolerances have been known to be tighter, requiring a larger amount of force to be applied.
Sometimes the parts require a bit of break-in where you would attach and detach the J-Arm in order to "wear it in." The Polymer on the J-Arm can wear down a bit on the aluminum mount interface. Try working with the J-Arm a bit more. But, if you are still having issues, we'd be happy to look at it. You could send the pieces in to our office and we can diagnose the issue and return you with parts that fit up easier. Just one of the headaches working with parts of different materials from different manufacturers...