(314729.112)I think it may be a little about his stamina.
For example, a player with stamina 3, his best time on court per week is about 55-70 min;
a player with stamina 7, his best time on court per week is about 65-80 min.
A player with stamina 9 can even pop his shape even he played for nearly 90 min.
So unless your player's stamina is lower than 3 or extremely high (9/10), I am pretty sure you are just unlucky.
btw last season I face this too, my star player stays shape 7 for nearly whole regular season average playing 75 min ( stamina 9) :(, but he finally has 9 game shape in the playoff :)