As you are doing snaplage, and not HMEC, you DO need to keep the moisture up, or you will have spoilage.
If it is building up behind the screen, and not between it and the cutter head, it is too wet.
If the problem is a few low spots, have you condidered cutting around them, and doing them later? That can get all the feed harvested at the right moisture, instead of some too dry, some too wet.
I've done that harvesting silage.