I've got a high traffic, highly active blog that gets fresh content posted, and lots of valuable user comments each and every day.

Back towards the start of this site, all new posts would index and rank within the top 20 on the first day easily for their respective keywords. Then one week something changed, and my posts started taking days, even weeks to index let alone rank in top 20.

I removed a language plugin which seemed to be making it too much work for the crawlers to search all my posts multiplied by each of the languages this plugin supported. I also added the deny to each of those language urls so the robots would skip them. I even jacked my crawl rate up in my google webmaster tools.

That helped get my new posts indexed almost instantly, but they still wont rank.

A new post will index, but can only be found by typing the keyword with my domain name in it. Even then I get older posts ranking above the new posts for that keyword some how.

It will take literally a month or more for that post to even show up in the top 100 with a healthy supply of backlinks.

It doesn't matter if I give it no backlinks, consistent healthy links, or straight up blast it, the outcome never changes.

I've been scratching my head for months trying everything to get this site back to where it used to be. I have a friend in this same niche doing pretty much the same thing, and his blog has only gotten stronger. All his new posts rank extremely well yet even when I out backlink him it doesn't make a difference.

Another wierd thing is that the main page ranks extremely well for its highly competitive keyword, and has for a few months steady now. Also, all the older posts that were made before all this started happening continue to rank very well. So the entire site is clearly not slapped or penalized by google, it just seems to be new posts.

Oh and this site provides very valuable information for its users, not just a thin affiliate site so I don't see why google would purposely hurt my site.

Any ideas?

Google changes or tweaks its algorithm every so often. Your rankings changed but you were not slaped. My guess is your on page SEO is less like by google now.

Make certain that the plugins are not confilcting in SEO. Use multiple content in your posts:written, vids, audio, pics, etc. in any combination.

I can't think of anything else.