You have to dust the entire environment of the bird and all birds that use that area, not just the one bird. Moving the bedding only moves the eggs and some still remain where the old bedding was.
If the parasite you have is immune to a particular pesticide (they do develop some immunity after exposure if the dosage isn't correct (just like with antibiotics) so you need to pay attention to the directions on the label and) rotate to another pesticide if several applications doesn't do the job.
Also pay attention to any other birds that enter the chickens environment, you can get an infestation cleaned up and have one wild bird deposit a load of parasites and you are back at the beginning.
Most dusts can also be dispensed in liquid form as a fine mist which is very good for covering large areas. Just be certain to remove all water in feed from the area while applying.