I've never been able to blow through check valves. Maybe I just have weak lungs, or maybe I just tried it with cv's that had too high a cracking pressure. In any case, the integral check valves in those JBJ-style bubble counters have always been garbage. If it's the silvery type, I have less faith in them than even the plastic ones from the chain LFS's.
However, my money's still on a leak. Break out the kiddie bubble blowing solution, or a couple drops of dish soap in a cup of water if you don't have the bubble stuff. Apply liberally. FYI, the plastic syringes from the pharmacy are multifunctional, and this is one of those functions. You don't need one, but it makes things less messy.