Hi, I just joined today however I'm very familiar with ear problems in Cocker Spaniels since I have been involved with Cocker Rescue for 12 years.
I agree with Marty, this sounds like an allergy to the ingredients in the Z/D.
Most Hills food is ...um...not great for long term feeding of Cocker Spaniels.
Most Cockers need a grain free food. She should be fed one with a protein that she has never had before. We feed most of our rescue dogs Dick Van Pattens.
Some are on the Bison, Fish and one is on the Venison.
Has cleared up ears within a month or so.
Give no treats..no vegetables. Use the kibble as treats or if you feed Dick VanP's food, they have a matching treat for each type..pretty much.
Dog's ear canals are an L shape unlike humans where ours are pretty straight.
Vets can do flushing to clear out the ears which will make it easier for you to continue with the cleaning.
We use a ProConcepts solution called Pro Otic to clean ears.
It is more tolerable than the Blue Power...and less messy.
Your original post was last August so I hope you have found something suitable for your dog by now.