There are several reasons that the results returned by SQL Search might not be as expected.

Index is out of date

If SQL Search doesn't return recently added objects, or does return recently removed objects, try re-indexing. On the top right of the SQL Search window, click Index, then Re-index. Alternatively, you can use Ctrl+Alt+D to re-index.

Results are limited to 500 per database

For performance reasons, SQL Search only returns 500 results per database. If some objects you were expecting to see aren't in the search results, this could be the reason. 

To see all search results, find the banner at the bottom of the search results panel, and click Show all:

Searching is filtered wrongly

If some objects you were expecting to see aren't in the search results, check the search filters:

Make sure that you're searching in the right server, database, and type of object, and that exact match isn't enabled.