Your dog probably has dust mites. The dog will lay under the bed and other places that you can not reach with a vacume cleaner. When the dog lays in her bed some of the mites will get off but will remain there so when the dog goes back to the bed the mites are still there. They can not be seen by the naked eye. You need to wash sheets,pillow cases ,and everywhere the dog has gone where there is carpeting,rugs,her bed,etc... Give the dog a good bath after you do this or the problem will remain. If you have carpeting in the bedrooms you will need to take the beds apart and vacume under them . Also wash her cloth bed and any cloth toys that the dog carries around. You should notice a difference in a couple of days. Look up dust mites if you want to read up on them. Good luck.