I had trouble with my Montesa when new but the chain was tightening up after a big hit. Cleaned up the factory grease from the sides of the cams, washers and spacers and OK. (leave the threads lubed though)
I also make sure the cams are oriented so that if they get hit with a rock they would turn in the loosening direction. Saves the chain and huge loads on the bearings.
Mark the position of your cams next ride. If the chain loosens without the axle moving then maybe time for a new chain.
By the way I found that the DID (non 0-ring) chain that I replaced the stock chain with, wore quite a bit faster then the OE chain. It was on the slightly worn sprockets though.