Don’t spend money on an SEo expert, I advise against it. Too many out there who really don’t know anything that would help.
There could be any number of reasons.
I would go here http://groups.google.com/group/Google_Webmaster_Help-Indexing/topics?hl=en .
Any kind of bought links, excessive link exchanges, etc can put you in the dumpster. Linking to bad neighborhoods can too.
There are a zillion things really that can cause this. You will learn a lot by just reading the above board every day for a while, all the stuff keeps coming up repeatedly so you will likely learn a lot quickly.
Google DOES have employees looking there, and if you ask a question you have to give a link to your site and be prepared for scrutiny.