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Often there is a tiny "mis match or oval condition at the front of the locks.
There are plugs in the end of the tailpiece tubes, and these also may have little "step" or misalignment that causes the boom to "stick" just before the tailpieces come out of the boom arm tubes.
Often pulling them apart (the tailpieces are at an angle with the centerline of the boom so either the boom head has to flex open a bit or the rear end needs to pull together a bit to allow the tubes to align.
Hope this helps,