The problem with these beasties is that they are very tenacious. Because they produce those things that look like little pieces of cotton wool in the teensiest crack in the most hidden bit of your plants, they can survive fairly sustained attempts to get rid of them. I used a systemic insecticide, and I made sure I sprayed my plants once, then once again six weeks later to make sure any that made it the previous time didn't make it the next. I'd been mealy free for about six months before I got sent a retusa with mealies... sigh!

Try cutting back on the watering quite a bit for four to six weeks, at the same time you treat with a good systemic (I like Marathon granules but they are hard to find unless you buy wholesale) and spray weekly with a hort oil. This should get rid of them, but keep a watchful eye for a return. I have some that took me over a year to get under control.