when you have no space where your actual (in hands) weapon could be put to, the game wont let you change it via hotkeys

example:

you have makarov on hotkey 1 and cz61 on hotkey 2, you have cz61 in hands but no space in inventory, because your slots are all filled

so no chance to put the weapon away, because hotkeys are not working when the engine could not put the weapon anywhere on player model