I have to believe that ivermectin either only works in certain regions or those who say it doesn't work got a bad bottle. It no doubt killed my mite and lice problem last year and I had a horrid case!
missypebble your description sounds like mites, especially because you haven't stated seeing them. While its dark, take a napkin/paper towel and wipe the bottom of a roost pole. If you see flecks of red you likely have red mites. If theres a lot of red, your infested! Permethrin is a quick fix and could be used to treat the birds and the housing, or you could try the ivermectin and see if it works for you. You still need to treat the housing though.
Do me a favor... next time you have one with lice, take pictures of the lice, give ivermectin, then post before and after pictures. Also include type of ivermectin used, how it was given, amount given and approximate weight of bird.