shiningly

Subdomains are not a must. However, your titles will kill you. "101" - it doesn't mean anything to a search engine. "tips", "information" -> those mean something. If you have products, make sure to add "buy" or "purchase" or any such keyword which screams "transactions". You don't have it right now. tl;dr: Help the search engines to categorize your silos properly. Same goes for subcategories.

Good luck.

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.