Are they using Ivermectin injections ? Did your vet do a skin scrape to confirm?
You need to be careful not to overdose them if you use a spot treatment (I used to use Xeno 450 on mine)
You can get mite flare ups if a guinea-pig is stressed . Is there anything like a fox worrying them?
I'd also change from sawdust , it can make the fur dry . Keep the hay , maybe just use newspaper .
I've used equine cardboard bedding and shredded loo roll tubes as a soft bedding . Loads of owners use Megazorb, it was 'ok' but not something I re-purchased .
Check your hay , it can harbour mites . (I had a problem with something -not mites , I think they were lice - on one of my piggies , when I bought hay from a particular store . You know the one . @Home )
Had to wash all my pigs , I used Lice'n'Easy and it cleared thankfully . I'd binned the hay wrapping so I couldn't get a refund .
Have you tried bathing them? There are some Gorgeous Guinea shampoos and melts . Their skin might be itchy if its dry and it may be a relief for them. (Of course make sure they're properly dry and warm enough after )
Mites are rough on a little piggie and they can get into an itch/scratch/itch cycle . Some piggies are lucky and never suffer them. Most piggies carry a dormant mite load that never flares up.
Some are prone .
(It was only because one of my guinea-pigs was a Himmy that DD noticed the infestation so quickly . She was worst affected her female cagemate had a couple , our Teddy boar was fine .)