First of all - don’t rely on Visual Studio AX SSRS previewer. It is a rather limited and, I would say, problematic tool. I have experienced different kinds of problems with VS previewer - starting from the fact that report is displayed absolutely differently from the end result and ending with the situation that it is almost impossible to preview some of the reports…
Therefore deploy the report and check the real result in AX. I know it takes a while, but this is how you can ensure that you are really on the right track.