How to allow Asset removal in the Control Panel for an Entry
Does everything else seem to be working? Maybe try removing the field from your section and then re-add it?  

You might also try clearing the caches: Settings -> Clear Caches.

Craft logs various things to craft/storage/runtime/logs/craft.log If there's an error of some kind, maybe with your database, it'll show up there.

If that doesn't clear it up, shoot P&amp;T a message and have them take a look at your install. You can do that right from the control panel.