A common ingredient of most fabric softeners is made from animal fat or lard. It has a tendency to rot in hot and humid climates. Try this first: Get a bottle of Washer Magic (found in the laundry supply aisle in your supermarket). Follow the directions on the bottle, using the hottest water you can get in the machine. Unless you have an unusual build-up in your softener cup/agitator, this should save you a bit of work in removing/replacing the agitator cap.