I'm wondering whether it might relate to whether you make your girl reach 100% excitement or not. Lately, as my stats have improved, I noticed I generally reduce the opponents' ego to 0 before my excitement bar ever reaches 100%. Add to that, that it's VERY unclear what each stat actually does. Which stats improve excitement? Which stat increases damage to ego?
IF excitement is somehow related to girl drops, then wouldn't that mean that eventually a boss simply can't drop a girl for you anymore because your equipment and stats are just becoming too good?