Fur loss in guinea pig?

[Nancyclancy]

We have 2 guinea pigs and have in the last week taken in two more.
When I collected the 2 recent ones, they were bedded on hay only, no shavings etc. The hay was also damp.
I bought them home, cleaned them out. Their cage is lined with newspaper, wood shavings, straw in the bed area. They have been eating well and had veg daily and plenty of hay along with their hard food.

Anyway, one guinea pig (the more nervous of the 2) is absolutely fine, but the 2nd (slightly braver) has patches of fur missing around her neck.
When we handle them, we've not noticed her itching or showing any signs of distress. I wondered if it was mites as one bit has a bit of blood. But if one had mites surely the other one would too??

When I first collected then I did notice once I'd got them home that her fur around her neck was a bit patchy so have kept an eye.
I'm just wondering if she's stressed, being bullied by the other guinea pig or whether it's her new diet? Any ideas?