could it be that your dogs never really wanted it? Staying 6ft behind for the whole of the run would of got me thinking. I bet someone will correct you and say buck not stag so thought I'd get there first lol.
just re read this, I'm in no way saying your dogs are shite by the way. But sometimes dogs don't feel it. Or if they've never seen them before could also be the answer
A dog should never back away from a deer as they are their natural prey. He should of run some whippets on it.