You can't just throw your thesis into a database-they cannot parse whole sentences properly.
If you want to know if family income affects children's school attendance in the southwestern United States, you need to break that up.
There are many words people may be using to refer to the ideas in you topic/idea. Brainstorm what those might be-- this will help you build a search strategy that will catch as much relevant literature at possible.
Look at your results, what terms are popping up? You might not have phrased it like that, but maybe that is an alternate term for the idea in the literature.