I would start by looking to be sure that there is nothing in the way, a rubber weatherstrip that has shifted or maybe someone threw something in the place where the top rests (behind the seat). You may have a weak ram.
If you don't find anything then take it down to the dealership and have the service guys give it a look and maybe they can suggest what could be wrong. This could give you a path to explore. Of course they will want to drag it into the shop, but if you tell them you are trying to diagnose it they may be willing to help at no charge.
Good luck to you.
Kate