What the heck's goin on?

Ok, so I’m working on my Tabatha…still, lol and I ALMOST have her rooted. But now I’m down to where there’s just a tiny bald spot in the back of her head, cause I prefer to work from the crown in, and now have ALMOST got her finished. Here’s my problem. There are spots where the hair does NOT want to take!! Places where the vinyl seems like it would almost get chewed up if I tried to hard, so I get a few hairs in there and move on, but really, in those spots, she could use much more. So what’s the problem? Why is it doing that and how can I get it to stop?