I found a lot of the same names were on subsequent giveaways for new books, so I've never done a second giveaway.
This is giving me a good idea, though. Perhaps after a year, I should do a new giveaway. I'll include a note that it isn't a new release.
I've found I get maybe 5 reviews for 100 copies out. They're certainly not guaranteed positive though, LOL, so if your ranking is good, I can see people not wanting to tempt fate.
The irony of book reviews is that once you're established, your fans will buy and review new releases, so you almost don't need the book bloggers/reviewers as much. It's like getting experience to get a job ... grrr hard to start cold!