I would say any player can reach any level. And at the end of the season I have 10 points added to my max (last time it went from 50 max to 60 max). I do not know the specifics of your stats but I would guess you have not added enough to raise it one whole star. Maybe, and this is a guess, you have added points to Offense, Defense, and Physical/Mental (as opposed to just raising stats in 1 category) but it was not enough to raise any category up one star.