There is a technique to fitting cork gaskets. Golden rule is NO SEALER unless absolutely nessesary. On a sump, fitting the gasket is a little difficult unless you use sealer to hold it in place though. If you use sealer, this allows the gasket to be squashed too easily and it slides out of the sides of the sump and splits as you tighten it.

All sealing surfaces need to be clean, dry and oil free (completely) and then you have a chance of them sealing.