I run a NetGalley co-op (have for two years). I will say that your success can vary based on a few factors:
1) Your genre. In my experience, Romance (all sub-genres), and YA that is skewed toward a female demographic do best. YA skewed male, thrillers, mysteries, and fantasy have not fared so well. Even women's fiction and cozy mysteries seem to be a bit hit or miss.
2) Do you control the requests for your listing? If someone else does, are they carefully picking which requests to grant or just accepting all requests? You'll do better if you select based on your own criteria and whether or not the reader likes your genre.
* Unless you're approving your own requests, I doubt your requests are carefully vetted. A new book can generate between 30 to 50 (more if it's a popular author) per day for the first week. That's a lot of requests to field if you're doing more than a cursory review of the requester. Not to mention, you probably know your own audience best as well as the particulars of your book (aside from just the surface details of genre and demographic).
3) Use of a preapproval link to preferred reviewers as well as following up with people who requested your book and (tactfully, gently, and politely) reminding them to review usually boosts results.
Hope this helps
