If it's the bungie friend system in game, then the game takes awhile for the friend list to update after becoming friends, and sometimes the friend request get put into a que, and is delayed sent. Also if your system is ahead in time, then all messages, request, invites, etc are sent into the void and lost. Simply make sure your systems time zone isn't messed up, and if the friend list is taking awhile to update, then try loading into a player hub zone, like the tower, and that sometimes helps update the list faster. If your friend request is caught in a que for to long, then the game automatically drops it. Simply have the person your sending friend request to, reject it, so you can send another. Also try to do in an a player hub zone, for some reason the game updates the bungie friend list more effectively when in a player hub, that's match makes with other players. DO NOT use a player hub like the helm, which doesn't match make, use places like the tower, or xurs treasure hoard.