1. Make sure it is not because of seasonality.

2. Access your site using a text browser like Lynx. Your first 5-15 links should be pointing to to content-rich pages which are unique.

3. Analyze the keywords. You said thin e-commerce sites are beating you. Are those keywords transactional or informational?

4. Continuing from point 3: Create silos. Separate your "information" and "commerce/transactions" neatly. Then, focus on the one which needs more attention. Add content/products and create more links.

5. Assuming you have analytics software installed: check your bounce rates, average duration/page, and pages/visit. See if something is amiss.