Have they closed/lined up correctly in the past?
If so, I'm guessing the threads have gotten gunked up with something, probably the female portion down inside the bow??
Or the mating surfaces have somehow swelled or gotten built up with dirt. It may be as simple as giving the mating surfaces a super light sanding and then do a trial and error check until it tightens. Want to be super careful though, don't want to end up with a loose/sloppy fit.
I would check those things and if that doesn't work, give Craig Warren a call (207-926-4133), he invented the system.