What I've done is to re-bathe the dog first. Before applying any shampoo, try to wash out with vigorous massaging/scrubbing their hair right down to the skin to loosen and rid them of the existing dandruff. Look at the shampoo you're using. Most of the dog shampoos out there if you read the labels say that they can be diluted down - which actually helps save money and tends to keep that from happening again. Dogs also don't need as much shampoo used on them like people do with soap or shampoo - so try to use it sparingly.
Good luck!
Good luck!