This happens to me now and again, too. It seems to be an issue with using any kind of overlay software (such as fraps) or having something else open while playing the game. I'm guessing it's to prevent any kind of duplication or modification of inventories, whether the user is trying to do that or not. Just try closing anything else you have running in the background and restarting KF2. If the issue persists, try restarting your computer, and if that doesn't work, the issue usually seems to solve itself if you just keep playing and wait a while. Fortunately, you can still equip/unequip cosmetic items through the "Gear" menu, even if they aren't showing up in your inventory. Hope this helps!