There are lots of engineers and hoo-ha's around at the conferences and maybe just maybe someone might take a look if you feel your site has been banned.
But that's not your situation. Your situation is that you simply don't rank as well. You'll have a hard time getting a search rep from any engine to give you tips on better ranking. It's not because they're mean, it's just not reasonable to request a search rep to give up some aspect of their algo.
Perhaps at a PubCon site clinic if a search rep is on the panel. Outside of that context, no one from the engines is going to divulge a secret aspect of their algo to help a stranger who isn't ranking as well this month compared to last year.
As for why your site may not be ranking better, you may want to think about what you have done to the site in the last four months that was different than in the previous months. The other thing is you may want to check your backlinks and see if you've lost important backlinks. Beyond that, algos change.
A solid site that has authority is going to last in the SERPs regardless of algo changes, with the ocassional blip once or twice a year. In general this is because of solid links that keep growing, solid history of not participating in the latest link scheme of the moment, and good outbounds to solid neighborhoods, and good content that is growing and attracting links- and that applies even to the ecommerce sites.
Granted, imo, Yahoo has a tendency to over-favor dot edu and dot gov sites in certain serps, a solid site is going to be able to ride above those kinds of exagerrations in Yahoo's algo.
Keeping in mind Robert Charlton's honest assessment that it's difficult to say in detail what may be wrong, the best that can be said is, if it's not something you did, it may be something you didn't do.