I think the pyrethrin products are most effective for the larger external parasites like the mites and feather lice...... not the scale lice.
At least those products never helped reduce the scale mites on my birds.
Sadly the scale mites are a true bear to get rid of.
Not being able to see the actual critters makes it worse in my book.
Just how bad off are your birds? Have you seen any improvement?
Pictures would be great.