some people watch porn vicariously. so if they cant picture themselves as the guy or wanting to be the guy its not enjoyable. i think its stranger to fap like some desperate onlooker but whatever...
Tell the chick sitting on your face to swivel her hips backward like she is getting ready to get it doggy style and that helps. Also having her shower up and wash just before is a great idea. To make sure she is a clean as she can be while washing putting a finger just a 3rd of the way in the pooper and checking for any bits left behind is something many people don't do. Not doing that is why after wiping clean you can go back a few hours later and wipe and there will be a bit left.