I have almost exactly the same problem. I beat the 25NL tables easily. But the 50NL tables are significantly tighter (full ring change from 45-50% see flops to 30-35%) and people are also much more aggressive.

I go from 10PTBB at the 25s to barely break even at the 50s. It is a small sample size and I think I do play a bit scared, but the difference does seem a lot bigger than people give it credit for.

I was really glad to read this post - I thought I was the only one with this problem.