When I send a message to most of my friends, it goes to 2 grey ticks (delivered) almost immediately. Then for most people 2 blue ticks once they've opened it, although some people have it set so it never moves off grey. All good, I understand.
With one friend, it stays on one grey tick (sent, but not delivered) for ages. Once it goes to two grey ticks, it moves to blue quickly and he usually responds almost straightaway.
Obviously, there are reasons I'm over thinking this, but what is actually happening? Why are these messages taking so long to be delivered?