If the disc material is too thick, you can start by using shims on the pressure plate, though I do not recommend it. To me, it sounds like hydroulics. If you did not completely remove the slave during the install (lines, etc) then I am guessing that you did not bleed the system. In this case, it is sometimes possible for the piston in the slave cylinder to move out quite a bit and inhale some air. I would try rebleeding the hydroulic system. If the problem still exsists, the seals in either the master or slave could be shot. The increased load on the components can destroy worn seals in no time.