My understanding is injuries early on and currently it's a lack of confidence in technique, he obviously has the physical tools to compete (he's a freak athlete, along the level of Armstead) but like almost everything else sports related the mental part is as important as the physical.....
But that is just what I've been hearing as a fan, probably more to the story, either way he hopefully makes some good strides to winning the RT job in the immediate future.....or he is probably gone.....