There can be a tolerance issue on the inlet valve, you sometimes have to try a BS005 alternative to get it to seal reliably, I recently had one in that would not seal no matter what ring was used, in the end I changed the inlet valve and that cured the problem.

I suppose if your air cylinder inlet bore is close to max tolerance and your inlet valve spigot close to bottom you could get a stubborn leak, after all they are made to production tolerances not absolute precision.

HW100 Tuning is a good supplier however, all the rings I have bought off him have been correct to model and size.