Woolly aphids can be very hard to eradicate and are a common apple pest. If you're attempting to use pesticides as a solution, you must cover the whole tree and really get it in under the woolly covering that the pest produces. Canola oil or other oil based organic pesticides are effective if you cover all the pests.
Unfortunately they do a lot of damage underground where it's harder to get to them. Again, companion planting to attract predatory insects is really useful in controlling them. Allow some carrots to go to seed each year (they're biennial) to attract ladybirds as well as lacewings which are the two best predators of aphids.
If you have chickens, let them forage under affected trees regularly as they'll eat the pests. Winter is an especially good time to allow them to forage under fruit trees.
Plant aromatic plants around your trees to repel the pests too. All of these measures in combination will help you control woolly aphids. It may take a couple of years to get bad infestations properly under control.