First, make sure he gets 48 minutes a week and that he plays those minutes in a position you train. I think that you should not train more than 2 positions at a time as well.
Second, remember that the potential does not increase how fast your player increase his skill. It only tells you for how long you can keep training a player. Once you reach a specific level the improvements will slow down for a player but even a 6th man potential will be capable of training for some seasons.
Third, remember that older players trains slower. If you compare his improvements in skills with an 18 year old it will be slower. I am not fully sure of this but at least in other management games training speed is strongly related to age.
Fourth, take a look at http://www.buzzerbeater.com/BBWeb/Forum/read.aspx?thread=381&m=1. Probably you already did so. If your player is raising skills close to what is written there you obviously train something which goes slow to increase like e.g. outside defense and there is no reason to worry.
Fifth, remember that for some skills you will improve slower if your player is either short or tall.
Maybe you already knew everything written above but hopefully it been to some help:)