Hmm... without seeing these spots first hand it is hard to say. Are the spots on the outside of the scales? Or are they buried between the scales? Have you just treated the snake the one time? Did you treat the reptibark before letting the snake back into the enclosure?
One thing I discovered first hand when trying to get rid of the mites we had is that they are resilient little buggers. They lay eggs everywhere - even between the scales of the snake - and not all mite treatments destroy the eggs. Repeated treatments every 3-5 days (depending on the directions of the product you are using) are needed to wipe out the infestation completely.
1) Allow the snake to soak in warm water prior to treating.
2) Use newspaper or paper towel for substrate as it provides fewer places for the mites to hide and/or lay eggs.
3) Be sure to wash anything that comes into contact with the snake thoroughly. (Hands, clothes, and anything else)
4) Treat any other snakes in the house to insure they don't spread.
and
5) Persistance - continue preventative treatments even after you think the infestation is gone. If, after about a month, you see no new signs of mites it is safe to say you have destroyed them all. I'd still recommend treating the cage as a preventative measure at least once or twice a year during the warmer months.
It can be frustrating and overwhleming, but in the end it will pay off. A lot of it depends on what you are using to get rid of the mites. The best one that I have heard of is called Prevent-A-Mite and works more effectivily then the big name brand treatments. I don't know if it is sold in stores, the one person I know who has used it religeously buys it through an on-line provider and has it shipped to her. If I can track down the name, I'll be sure to post it here for you and anyone else who might need it.
Jenn