It doesn't look like anything medical or pest related, just thin fur. Perhaps your lines aren't as dense as others, or perhaps the breeder did some cross breeding at some point to compromise the density of the fur. They still may bulk up after their first molt too, one of my last litters did, though they never got any guard hair. Perfect for spinning, but no good for show.
If they don't, then you can start holding back some of their densest kits and start breeding out of those to increase your stock's density. It could also be that if you didn't get to pick each kit that these were the least dense of the litters since it can vary within each litter. It's hard to say.
I would be somewhat skeptical of anything the breeder told you (if she had mentioned they were the best or, pure bred, anything to that effect) since she was willing to let them go so early. In my mind that says she's either just honestly ignorant and doesn't know better or she just doesn't care and is in it for the money. We get a lot of both types in my area with angoras because they're cute and fluffy and they don't realize what goes into them and they just see it's one of the breeds that sells for more. Its the main reason I've become so strict about who I sell to. I'd rather eat them than have them go to a bad situation.