Make sure that the blade and housing are both clean and secure. if there is not 100% lockup (tightness all around), there could be a small variation. This will cause a thick slice or blade to angle deeper into the tissue, potentially snapping it off the cassette.

The problem may not with the blade or microtome, and it may be with the tissue embedding and paraffin block preparation. After upgradtion in alcohol series, and prior to wax infiltration if the tissue gets over exposed to the organic solvent (benzene/toluine), there is a possibility that the tissue gets harder and makes wax infiltration improper. If you try to cut such a block, first few wax sections without tissue will be OK but later ones with tissue, either get crumpled or tissue get powdered or block get dislodged from cassette as you are experiencing.

Make sure that the blade and housing are both clean and secure. if there is not 100% lockup (tightness all around), there could be a small variation. This will cause a thick slice or blade to angle deeper into the tissue, potentially snapping it off the cassette.

Something is loose.

When you have variation in how the block approaches the knife edge, it can 'bite' deeper and impart too much force onto the entirety of the block and pop it off of the cassette.

It could be the stage, the lock for the knife, the clamp for the cassette, or even something interior to cause it to slip a gear and jump 20 microns for no apparent reason.

Such problem occur when (1) you mixed paraffin wax of different melting points (2) you don't allow the paraffin to set continuously after embedding (3) particles of solid paraffin is trapped in the melted paraffin during embedding (4) you failed to clean and lubricate all the surfaces that are supposed to be lubricated (5) the specimen holder or knife holder is/are not tightened and (6) your knife become blunt. When any or more of these problem occur you can have such difficulties.