I was wondering the same thing... I've just moved up to NL25...
I was making good progress on NL10 but when i hit NL i promptly lost $30, in my first 1000 hands, there were a few suck outs against me, a couple of big mistakes, and also it seemed all my raises in late were getting reraised, i thought people were stealing from me so I called a couple only to see monster hands...
It knocked my confidence a bit but the SSS system says to stay at the level until you get to 30 buy ins of the level below before you go back down, so i've stuck with it... and I have now stopped the rot, though without any great progress, but i think it's a case of things just not going my way... I've only played about 4000 hands at the level so it is early...
So far in my small sample of NL25 i'm finding players a lot tighter pre-flop, but then after the flop they are much more aggressive ... which makes it a lot harder if you don't make your hand, which is what seems to be happening to me...
It's difficult to hit a down swing straight away on a new level, it really knocked me back, forced me into making errors... I hear it's not uncommon though.. so i'll stay positive..
At least now i see why we should only move up levels when we have at least 30 buyins.....
Another positive is i'm making good progress on the $100 bonus on PP...