broadcast message hasn't made it to one recipient even though he has been online multiple times

Hi, I sent a broadcast out today and the message went through to all but one contact. All day it has shown the one blue tick but never changed to two. I have checked that I can still see his last seen time, profile picture and status and there is no problem there, I have noticed he's been online a number of times since this morning, so I don't think I've been blocked but the message still hasn't updated to two ticks let alone showing the blue ticks. Any ideas as to what might have happened? Why has it only happened to one of my contacts?

Oh! and I have noticed that it has been taking progressively longer to send messages to this particular contact over the course of a few weeks. Any help would be greatly appreciated.