Did you forget to loosen the stud lock???
They are these little screws inside the post (the black post thing you ''twisted'') whichwhen tightened, push down on the bottom of the post "thing" cavity which firmly holds the post "thing" in place.
They smallest allen wrench included with your Jem will loosen the stud lock.
If your local guitar store doesn't carry any posts, you need to order the parts through Ibanez. Lucky for you too! Ibanez (or whatever company is was) recently came out with new Edge/Lo
Pro Edge posts.
Check out the forum somewhere on jemsite. *APPARENTLY they're better.