If the dog eats or drinks from a plastic dish that could be the trouble. Many dogs have allergies to plastic dishes.
I'd switch to stainless steel.
Here is some information on plastic dish dermatitis:
http://www.fda.gov/fdac/features/2004/404_pets.html
"A reaction to physically touching a substance is called contact allergy, the least common type of allergy in dogs and cats. Contact allergens include grass, wool, and plastic. Jeffers occasionally sees dogs in his clinic with "plastic dish dermatitis," an irritation to the skin on the nose caused by a reaction to an antioxidant found in a plastic food or water dish. The condition clears up when the pet is switched to a metal or ceramic dish."
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I've personally seen conditions where the lips, chin, neck and cheeks become irritated from the plastic toxins, especially in flat faced breeds or mixes.