In Grave Peril we see Harry's magic turn against him for (thrice?) breaking his promise to Lea. We also see him perform a truly incredible feat of magic after Susan bargains for temporary relief from his condition.

My thought is that Harry breaking his promise to Lea not only caused his magic to turn against and harm him but also stunted his magical ability (power/skill/etc.).

This would explain how, debatably, Harry grows stronger throughout the series. Susan's deal still lingers in Summer Knight and Death Masks onward his ability is slowly returning from repaying his debt.

So discuss, poke holes and tear apart this theory at your leisure.

 

If I understand this correctly, when he broke his promise to Lea it was sort of like when a wizard swear by their magic, if they break the promise they lose some of their ability. So yes, you are right in thinking that.

However, I don't think that this has anything to do with Harry's increasing skill. If you notice he has been getting stronger then he ever was before breaking his oath. He is simply getting stronger because he is gaining experiance.

 

Huh, I think Butcher would have dropped a few hints or included Lea a bit more throughout the series if her and her bargain had had such a notable impact on Harry's power during the following books. I'm going to go with Novera on this one.