Does the system have fluid? Is there fluid leaking from the master cylinder? How did you "check" the slave cylinder?

I have replaced a clutch before when it didn't need it because I was convinced that the hydraulics were okay. I have also had a finger break off on a pressure plate that caused weird problem, but that is certainly not a typical failure.

If the pedal goes down and won't come up, I say the odds 98 out of a 100 that the problem is hydraulic related.

Good luck,