What injection did the vet give you and what breed? Lice are very common at this time of year, and yes some goats are more susceptible than others - even if they share pens etc. Lice powder will not deal with them, also ivermectin injections have to be given more than once, it is either 7 or 10 days after first one, or you may even have to continue. It should deal with lice (both sucking and biting ones) In the meantime I would introduce a Zinc mineral lick (one of the Rockies is high Zinc, but you can only get the big ones - mine are hung up with thick fencing wire through the central hole). If he isn't used to Rockies, you could also boost him directly with a sheep mineral drench with added copper, his stable mates could probably also do with a top-up.
I take it the vet ruled out scab.