I've never seen ZSS, but there's more to a good zine review than how frequently it comes out or how many zines it reviews, such as:

* Quality of the writing. Is it coming across as unbiased? Are the reviews informative? Is the writing engaging? Does it get to the point quickly?
* Professionalism. Are there typos, or is the copy fairly clean? Is the layout clean-looking? Is it easy to navigate through?
* Eye-grabbing. What are the covers like? Are they colorful, with lots of information on the outside? Is there TOO much information on the outside, making it look cluttered? Most importantly, do they please the eye?
* Marketing. Where are you selling this? Are you targeting the right market? Are you letting the people you're reviewing know that they've been reviewed? Many zinesters will be interested in reading reviews about their zines.

If you have all those bases covered and are producing a flawless masterpiece of zine reviewal, and you want to know why ZW sells more copies than yours, it's likely that name recognition is a key factor. But my guess is that there are more elements to it than that, and you should get some honest opinions about your product - from people in your target market, notably subscribers of your competitors.

By the way, did you get any feedback from the publishers who weren't interested? That would certainly be great feedback to have.

Good luck!

~MariNaomi