One has to remember, there are two different kinds of spotting that you can find on glass. One is hard deposits from water, the other is actually etching into the glass. The water deposits can generally be removed chemically with vinegar or one of the other specialty liquid products. The spots that are caused by physical etching of the glass can only be removed physically. No amount of liquid chemical is going to take them out. They will require an abrasive.
My advice has generally always been, start with the weaker (liquid) solutions first. If that doesn't work, move on to the abrasive removers, which is what the Duragloss, and Invisible Glass (And many others) are.