I'm writing an essay on the case of Plessy v. Ferguson and I have an idea of what I want to talk about. My problem is that no matter how much I research it I can't find anything to build a thesis off of that would have enough information.
So broaden your research one step further.
If you wanted to talk about its effects in southern cities, maybe compare its effects in southern versus northern. If you wanted to talk about public protests surrounding the decision, maybe talk about "reactions" more broadly.
It's better to use the information that you have, THEN arrive at your thesis based on it. Coming up with a thesis first (a hypothesis) is hard in history, because you're right--sometimes there isn't enough information, or enough information that you have access to, to argue it.