I can see the problem straight away.................... Butts are manufactured with the male part of the butt socket very slightly tapered and yours ain't! The result is that while on a normal military type butt, the tapered part of the butt tightens up in the socket as the stock bolt it tightened, yours isn't, so it can't tighten up. Simple as that. So as soon as the front part bottoms out, that's as far as it goes and if it's loose, it stays loose!

Some later No8 rifles had the same problem. Next question....................