Unless it's different from every other garbage disposal I've ever worked on, the locking ring collar only need about 1/3 turn and the disposal drops from the sink. Did you turn that or are you just turning the disposal? Sorry if that's a silly question, but it's the only thing I can think of.
In case you're not sure, the ring collar (not sure exactly what it's called) is about 4 inches in diameter, sits between the disposal and the sink drain basket, and has 3 "curled back" tabs in which you insert a screwdriver or similar so you can turn it. Disposals come with a tool for this, which looks like an allen wrench, but is almost always lost after the installation.
I hope this makes sense, hard to describe with words.