Lots of us have a hard time getting rid of scale. It seems to be the hardest pest to eradicate. I posted the following in the orchid forum but i think re-posting it here is appropriate:

I am very pleased to announce that I FINALLY got rid of scale in my plantroom after a two-year long battle! I didn't want to use poisons so I tried all sorts of natural home-made sprays, hand-picked them and tossed away plants that were severely infested. But I drew the line when I found them on one of my orchids. Those suckers traveled across the room to get to it. I used Bayer Advanced Insect Killer systemically. It worked - I won this war!