Did you check expiration dates? I was buying 20-ounce bottles of Coca Cola (this was several years ago) from a certain coffee shop near work, and they often would be flat. I finally saw the expiration (or sell by) date on the bottle (not easy to see that dark grey print when it's full of pop!) and noticed that the pop was expired. Even if it was by just a month, the pop was flat.

I called Coca Cola and the problem was immediately taken care of. I hope the same happens as a result of your letter.