You must evaluate what your competition is doing to outrank you. Check backlink profiles using tools like AHREFS, OpenSiteExplorer (SEOmoz) and/or MajesticSEO. You can see useful info even with a free membership.
Perhaps your competitors are outdoing you, either with natural link bait or SEO?
Those links from free blog sites can be helpful, but they also must be quality, relevant sites with their own inbound-backlinks to have any link juice to pass along to your money site. Diversity of inbound links from various IP addresses will help a bit. Remember, relevance and quality are more important than quantity of backlinks.
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SERPS are always relative to the competition.
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Also, do any of your feeder-type blogs have anything better than PR-0? It may take 1000 links from PR-0 pages to equal one link from a PR-3, in terms of link-juice... But relevance is also important.