User blog comment:B214/Volume 21 info & spoilers/@comment-205.197.242.172-20160708201350/@comment-24.113.252.213-20160710212533

I HUGE factor in that fight was Diehauser's "Worthlessness" ablility. Whenever Issei used [Boost] to increase his power, Diehauser used his ability to render it "worthless".

I'll give a better explaination:


 * Issei used [Boost] to increase his power.


 * Issei tries to punch Diehauser with his increased power.


 * Diehauser blocks Issei's punch, activating his "Worthlessness" ability.


 * The power increasing ability of [Boost] was rendered "worthless", making Issei's power go back to where it was before he used [Boost].

Diehauser doesn't negate Sacred Gears themselves, like Rizevim does, he just negates their abilities. So as long as Diehauser keeps spamming his "Worthlessness" ability, It won't matter how many times Issei uses [Boost], his power will stay at his Base+Scail Mail.