just keep scraping. If you're looking to get 15k+ comments then an AA list might be worth it. Otherwise just keep harvesting for URL's. Use a custom footprint and search for "power by wordpress". Also once you get some comments to post make sure you save those successful postings so the you can reload those back into the harvester. Just do a "trim to root" so you can search for more blogs on the same domain. I'm still pretty new at scrapebox but I've been able to get a decent amount of comments to stick like this.
One other thing, if the comments are moderated you might want to vary your comments to match the blog topic. For instance, I went onto google trends the other day and I noticed "ryan dunn" was a hot topic so I harvested a bunch of blogs about him. Then I pulled like 10-15 tweets about him and used those for my comments.
I don't know if this is stuff you already know or not; again, I'm pretty new at this too but it's been working for me.