Team not always joining game when in sqaud

When we finish one game and go into another. You sometimes find some of your group don't go in the game. It stays stuck on the progression screen and says matchmaking even though nothing is happening. Once match making completes. You as the sqaud leader is taken in the game, while your friends still appear in the lobby! If I back out the game and restart the game they still don't join. It's like the session has gone stale! Even though we've all restarted the game!