Here is another possibility or two...
When the viewer opens is does the report load but shows no data? Typically this is a white background. If so I would check the data connections and queries. Generally when this happens to me that is where I find the solution. If the backrground is white, generally I find that to mean there is no data being returned.
If it is a gray background you likely are not connecting to the database or the report itself is not loading. Brian's merge module solution might resolve the problem.
I would also look at the deployed app to make certain the database permissions are correct.
Regards,
John W.