Does success ever go down?

I know it's fixed at 75% at similar...but does it ever go down? It kinda stuck me as odd as I can put, literally anything to sleep with around 200cp...

200 or 2,000? I've put Cromm Cruaich (10k CP), Hard Doppelganger (40k CP), and Elite Doppelganger (60k CP) with just one try.

I thought it was established that the success rate is fixed and is not CP-based, contrary to what the in-game description says? In any case, (and you know this was coming) lets see what JP wiki says.

"同レベルの敵の場合、睡眠確率75％" Or "If enemy is similar level, Sleeping Chance is 75%"

I dunno x.x...I don't see the reasoning behind anything regarding 75% sleep chance if I can just sleep anything with 1cp ._. Maybe it was a beta thing or my music buff effects are stacking the odds. I ahve around +60~

It's not CP based at all. I don't think I've met anything that CAN'T fall asleep with a 75% rate since it usually takes only one or two tries for it to work.