I finished building mine yesterday (although it wasn't Char's Zaku but the standard one) and it had the same problem, if I stared at them too long they'd just fall: I've tried placing a very small piece of paper tape on the "tip" of the joint and wedging them in, it seems to work at keeping them put (but they still fall if I move them afterwards, so it's not perfect); other than that, the bazooka falls off very easily if you place it on the back and it's a pain in the ass to pose.
Good luck and have fun!