anallawalla - 10:41 am on May 22, 2011 (gmt 0)
I don't think the same server is the problem. I don't think interlinking is a problem, as I know a few interlinked sites owned by one entity and on different servers and they haven't suffered. I also know a big magazine site that was hit by Panda and whose only fault is to have lots of copied press releases. It links to a few other compay-owned sites.
My assessment is that you have catalog sites with little bulk on each page (just my guess, as I am trying to imagine sunglass sites). So do your competitors, who are still ranking. In that case, look for big-hitting factors, such as backlinks. What is the quality and quantity of your sites' backlinks, versus those of the competitors? Do they have links from the manufacturer and you don't?
Also critically compare their sites with yours - do they have the word "sunglasses" (and synonyms) on their descriptions? It is common for catalogs of a single family of products to omit the actual noun they represent. Any other missing elements?