loaded crossbow sometimes do not fire

A loaded crossbow will sometimes not fire anything even thought it goes through firing animation. no arrow leaves the cross bow or impacts a mob. I have not been able to duplicate on demand but this has happened with plain cross bows as well as enchanted crossbows. this has happened when switching from a sword to a loaded crossbow in the hotbar or when switching from another crossbow. usually this happens when already in combat with a mob and not the first shot.