New Clutch Slips After Installation
  Adjustable Clutch Master Cylinders   Clutch Slips  If the clutch is cable operated then you might need to slacken the cable. Clutches are the opposite of brakes, as they wear brakes get slacker but clutches tighten up. If there's insufficient slack in the cable then this will prevent the clutch from fully engaging.    If it was slipping because you rode it, the plates would be glazed. Good plates would means that they did the job wrong. A greasy flywheel maybe. The pedal is stiffer because they used stronger springs when they did the job. If it's mechanical linkage, they might have left it too tight.   poor job of replacing clutch assembly ,if  only replaced the disc and not resurfaced the flywheel and replace the pressure plate. Once they get glazed form slipping a new disc doesn't help very much.  Look at your old receipt for the work, it should list the parts:  clutch disc  pressure plate  pilot bearing  throw-out bearing  pressure plate  job-out...resurface flywheel  ...