Hey equinewiz Lice and mites are sadly pretty much everywhere in the environment and can be bought in by wild birds and I think even rodents.
It was just a suggestion as something else to rule out as a cause for their not roosting inside.
Something else that comes to mind is that they are not...
One reason chickens stop using the roost is mites. They would rather roost outside than get little bites all night.
Just a thought, but have you checked for mites or lice in the coop?