FetchXML works fine in Organization.svc but doesn't work in WebAPI

Hello,

I seem to have some fetchxml that is working inside of the Organizations.svc (SOAP?) using XrmToolBox. I take this same fetchXML and i try to apply it to the Webapi using the /api/data/v8.1/$batch... Doesn't seem to work at all. 

Is there a reason why fetchxml doesn't work consistently?