Assuming new clutch disc, you have the wrong slave or wrong throwout bearing or wrong pressure plate
If you have an old clutch disc then its too thin, worn out
The distance between bellhousing, where slave is attached, and the pressure plate forks is the problem
pressure plate||throwout bearing||slave||bellhousing
When you put transmission back on did you have to use the bellhouse bolts to FORCE the bellhouse to be flush with engine?
Bellhousing should just slide right up to engine, flush, no need to force it on by tightening bolts