If they keep coming back with the formula you have been using could be the hatching of the eggs or the formula is not for that particular critter. I would use Neem Oil an see if this does the trick. Use according to directions and I always make it a point to go back after 14 days to re-treat an infested area and wouldn't harm to treat the area around the infestation. Preventative thereafter with a weaker formula. Personally I have not had the need to use Neem Oil but a very popular product and works wonders from what I am told.

You might want to check this out...Insect and Arthropod Pest Identi

I've seen these little miniture bowling balls in 1 of my frog terrariums, not sure why there are in 1 of not the other - 'cept the moisture level is higher with the one that has mites, since both houses have wood.

There is a spray you can buy at the pet shops to rid of these little critters. It's animal friendly since this is what they use to clean the (reptiles and such), dunno if indeed it's orchid friendly - But I don't see why it wouldn't be :hmm