I assume that you have tried pulling the "[censored]" out of the luff tension?
If your cat has been sailed as a sloop you may find it necessary to have some of the luff round cut out of the main to be able to sail it efficiently as cat rigged, so that you tune the sail to the mast which is always a better result than tuning the mast to the sail (within reason) To "prebend" a mast to flatten a main is OK within reason, when it requires that "major" prebend is necessary the better option is to reduce the luff round in the sail cut.