Since this is a continuous problem...(you indicate it "keeps slipping" Tried a new one with same results?)

Possibably something is wrong with the machining of the crankshaft. The work around is to take a file and make the split in the tapered "collet" wider, allowing it to seat better around the crankshaft.

The taper inside the "flywheel" (thrust washer) could be machined too large (OPPOSITE PROBLEM from above!) The fix here is one or more of: 1) replace the thrust washer. 2) file off the top (narrow end) of the tapered insert. 3) put a shim washer (usually a glow plug washer fits smaller engine crankshafts for this) on before the tapered piece.

If you don't know if its the taper not gripping the crankshaft or not gripping the thrust washer.. stick the glow plug washer on before the taper insert... you'll know right away. (assumes the crankshaft isn't tapered... and has a definitive step in size that the taper piece rests against.)