there could be any number of causes for that, as I'm sure u already know.
IMO a good exit is "direction", "when you roll on", and "how hard you roll on" in order of most important to least important. most times I pass bigger bikes up the inside on exit, its because I had more direction. my bike is pointing toward the exit a lot sooner, so I accelerate sooner and harder.
spinning the rear tire shows your limit of drive traction and its not ez to learn how to spin it. u are going to have to push your limit. u just want to do so safely. absolutely make sure that uve already "finished" the corner before accelerating. absolutely make sure that u can and will subtract lean angle BEFORE accelerating. when u are sure of that, go to WOT in the same smooth but quick manner, just a tiny bit sooner. if u find yourself running wide, u went too soon. if the tire slides, u didnt subtract lean angle. but if u really did it just a tiny bit sooner, ull eventually creep up on that nice smooth tire spin.
ideally, u want ~10% wheelspin for maximum drive. but thats pipe dream stuff. u can win Expert races without any wheelspin if everything else is on point. but u probably still want to know where that limit is to remove all this doubt about drives. plus, this "upper limit" of drives is useful when working on mid-corner and braking.