Have you tried running a trace to see what SQL SSRS is actually running? If you have parameters involved, it may be it's passing a value you're not expecting and returns no results.
Hopefully running a trace will show quite easily where it's going "wrong".
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does :-P
Please always remember to encapsulate your code in IFCode Markup. For example [code=sql] [/code]
to read Jeffs Guide on how to post SQL questions, and get swift and helpful answers from the community