It seems odd that he wouldn't start any game, I could see if you came out in a formation without a MLB occasionally but all year is odd. Look at the first play of some of your games and see if there is a MLB on the field, and if there is maybe you have someone in your overrides? Another thought is that the MLB is on the kick coverage team and is fatigued from that play, but I would be surprised if that's fatiguing him enough to have him off the field on the first play (unless your fatigue settings are set too low)