I know that this comment will be greatly frowned upon but oh well here it goes anyway. If the problem is determined with the wood and not a warped plate, find a competent smith to scrape away some of the inner in-letting for the locks and bed them properly with one of the many compounds available, the smith should know what areas to relieve to allow the locks to function properly. If a good job is done no one will ever know without removing the locks and bedding compound is far more stable than wood. Of course it has worked this long with out an issue so bedding is probably overkill, however it should give years of service to the gun without a hitch.
This is an excellent solution, IMO. I have a Pedersoli muzzle loader that I shot, quite a bit, in NMLRA vents. Over a few years of frequent cleaning and frequent disassembly/reassembly, it became fairly easy to tighten the lock plate screws a bit too tight. Doing what Steve said above, worked fine for me. I used a few small brass finishing nails, to determine the depth of the bedding(Brownells) and the lock plates now have a stable place to seat, without binding anything.
