Custom element type defineSearchableAttributes
Craft will only extract the search keywords for an element during the element saving process.

If you have existing custom elements already saved, then you go in and add the defineSearchableAttributes() to your element type, you'll need to re-save the existing elements in order for their search keywords to be indexed.

Or now that I think about it... the easier way would probably be to just run the "Update Search Indexes" tool in the Control Panel under Settings->Tools. It will do the same thing for you.