Cant help feeling your barking up the wrong tree here.
I remember Jason Flowers from when he broke in to the first team, to when he left to Salford I think it was.
Flowers was one of the best defensive fullbacks in the game at that time, his one on one tackling was exceptional, as was his positioning and ability to take the high ball. However the reason that Jason was never world class was quite simple, he lacked any kind of pace.
I dont remeber Jason being tipped to be a GB fullback by many people during his career, as there were several fullbacks during that time that were nearly as good as Jason in every department, but were also gifted with bags of pace, an invaluable asset for any attacking fullback.
I think the reason he was overlooked by so many coaches at international level bears this out. That is not to say Jason wasnt a terrific player, because he was, I doubt if you will find any Cas fans who watched him turn out for us who would disagree.
With all this in mind then how do you feel he underachieved? I must admit to being slightly puzzled.