I think many people in the comments here, and most users, are missing that you index a SMALL subset of the web. This leads to people running a default test search, finding no results, and concluding your search engine is bad, and leaving.

While you imply that in the search page, obviously it's not clear enough.

Maybe add "this search engine only searches a small set of user submitted sites. Click <here> for the list. Or <here> to add your site."