
Sounds like this is an ongoing problem, & you need to find the cause, before you can get a cure.
You need your vet to refer you to a skin specialist / dermatologist, who can assess your dog, they'd need to get a lot of background info from you & history about the dog. ie bedding / diet etc.
If nec they can do a work up on your dog & run a thorough skin test, this involves shaving the dog & injecting the dog with lots of different things to see what the dog reacts to, but usually prior to this more basic tests are done. Including food trials. This would mean feeding the dog on a strict diet for a few weeks - but no other food at all is to be given during this time. If things improve, problem solved & it's a food related problem.
Unfortunately westies are prone to skin problems / allergies.