Dogs with ear flaps that go down are more prone to ear infections due to more heat, therefore more moisture, which can cause yeast. Also, their ear canals is where ear mites can live, lay eggs and cause irritation and itchiness like yeast as well. Ear infections and yeast in general can be caused by moisture, environmental issues, allergies to yeast in food or other ingredients in food, dust, carpeting, carpet cleaner, pollen, weather changes, detergent, fabric softener, bleach, or a host of other things. Also, depending upon how much hair grows in their ears the shape or size of their ear canal, can make them more or less prone to ear infections, because each dog is different. I personally use "ALL" detergent recommended by dermatologists, allergists and pediatricians that is 100% free from perfumes and dyes. This particular detergent is great for dogs or people with skin sensitivity/issues and all types of allergies. When your Shih Tzu is groomed, sometimes the groomer pulls ear hair. While this keeps their ears cleaner and easier to treat, it also can make the inner ear skin raw and itchy when it is first pulled. If a drop of water gets in their ears when grooming, that also can cause a yeast infection. It is important to be aware that any number of things can cause your puppy or dog to have itchy ears. If the ear infection is not obvious, sometimes your dog or puppy can act depressed or walk funny from a deep inner ear infection. Do not automatically assume they have some sort of neurological damage or an illness. Immediately book a vet appointment and have your vet check their ears first with an ear cytology, to rule that out before alarm or panic sets in. This is more common than one would think in the Shih Tzu, so rule that out first.