My son has the same issue. Once he went to play summer ball in NY and they looked at him and didn't think much about his bullpen, but when the first game came around and he struck out all 3 in the 9th (he's a closer) they were like, Wow, how often can you throw? Can you throw every night? The adrenaline just kicks in during the game. A bullpen is for warming up and stretching to him. Games are just different. He's had a great college career.