if you have a weapon on your shoulder and one in your hands, you have to set the one down that is in your hands to do stuff. however, it will randomly not show up on the ground or anywhere by you. but, if you leave and go back a couple of minutes later, it will be there. it does not matter if you are outside or inside or what weapon was in your hands. This seems to be extremely random and mostly only happens to me on non-persistent servers.