I have had this problem as well. Just in my last game my PG was set as starter, sub, and reserve but played only 35 minutes in a 70 point blow out. I had my coach strictly follow depth chart and let them play through foul trouble. My PG hasn't had issues playing 48 minutes in previous games but everyone once in a while I'll have my trainees minutes severely limited as starters.
Also there stamina is between average and proficient and their game shape is either strong or proficient.