Weapons aren't unlocked even after completing all Bell Towers

There are several weapons that unlock only if you progress through the main campaign. There are 17 belltowers, make sure you've visited them all. The stores also offer you some weapons so you definitely want to check them out.

This is a not-uncommon bug with Far Cry 4. Sometimes completing collectibles won't award the achievement or weapon it was intended to. In my playthrough I wasn't able to collect the achievement for acquiring all of Mohan's journals, despite otherwise 100%-ing the game. There isn't a workaround to this that I've found. Even uninstalling, reinstalling, and replaying the game didn't fix the bugged achievement for me - and I suspect it wouldn't unlock the weapon for you.

I'm sorry, but I don't think there's anything you can do.