Pouring them once for lice won't always eliminate lice. Don Bliss, one of the leading parasitoligists says, "If you want to kill lice, you must pour them twice, regardless of the product you use."
I know that some avermectin companies got into trouble awhile back and the government made them pull their lice guarantee because it was bogus. You don't see the claims anymore guaranteeing "6 months lice-free" etc.
The way I understand it, there are biting lice and the eggs. Pouring will get the biting lice, but not the eggs. It takes two weeks for the eggs to hatch. You need to go back and pour in two weeks to get the eggs that hatched.
So, it could be lice that are bothering your cattle.