As you mention 25nl is this on
Pokerstars?
Are you playing 6max or full ring?
I suggest you have a real close look at the stats comparing both levels to see where the differences are. There must be some difference otherwise you'd be beating it the same.
If you are struggling at 50nl 6 max then why not try full ring for a bit.
Happy to look at your stats. I play up to 50nl.