Would you be able to post screen shots of when the button is greyed out and when it is not?
Hi Matthew -
Partial guesswork on my part, but I suspect you are inadvertently creating a Calculated Filter:
A Calculated Filter is essentially a "Sum of a group of sums". You can read more about it here:
Try adding the dimension in your filter to either the Level of Detail shelf and/or to the Rows or Columns shelf - if you do so, you should be able to "View Data" again (note how the "Sigma" symbol is gone):