My dog has loads of white bits in her fur is it dandruff?

i noticed 2 weeks ago that my schnorkie (schnauzer x yorkie) has loads of white bits in her fur and it is really bad. i brought some dog special tea tree shampoo and i used it twice in a week and it still hasnt gone. what shall i do? do you think i should go to the vets and ask him what it is or do you think i should go to the dog groomers and ask her. please help me