I was playing Brink, and I was doing random campaign levels. Every time I beat Black Box, all weapons and unlockable weapons I have other than the starter ones disappear. I made a second account, but once again it has the same problem. My third account, the one I mainly played on, is a high level. I havent played on it because I dont want the weapons disappearing. Please help!

It's all dependant on your 1st character, the guy in slot 1. If you unlock all of the weapons with him, they remain unlocked for all other characters. But if you unlock all the weapons on anyone OTHER then character one, none of the other characters will have them unlocked. So I'd suggest you level up the guy you have in slot 1, your problem should be solved then.