boy oh boy have I had this problem .. but with my shih tuz and Laso not my yorkie.
I have read a lot of info about what to do to help on this website and from my vet and just trial and error.
One-- the food. A lot of dogs can not eat corn. or wheat.
I started give omega 3 oils-- fish tablets or samon oil. There is a web site that
www.dinovite.com -- check it out -- they sell supplements that really work!!!!!!!!!!
two -- I also read on this site about adding apple cider vingar to water.. it has to be organic with the "mother"... I have been using for about 10 days and I can see improvement over and beyond the other supplements. I really started for the tear stains on my white shih tuz.
I have had shots for my guys and it did help..for a while. The vet also said it could be flea bites. They don't have fleas but he said when they are out and about they may get a bite. I not so sure if I agree.. but??
Bailey will chew the hair off his paws if I don't keep up the supplements so I don't think that is fleas
Good luck.. I know you will get good advice on YT