A few different folks have reported to me that when accessing Google Postmaster Tools, they were seeing this message being displayed instead of data:
No data to display at this time. Please come back later. Postmaster Tools requires that your domain satisfies certain criteria before data is available for this chart. Refer to the help page for more information.
What criteria? The most common one is that you must have enough sending volume to merit data being displayed. In one case, the deliverability consultant knew that the client was sending 50,000-200,000 messages to Gmail subscribers daily, so that can't be it.
It turned out, if you enter your domain name with any capital letters, Google Postmaster Tools won't display any data. In other words, if I enter my domain name into GPT as SpamResource.com, instead of as spamresource.com, no data will display. The fix is to go back in, remove your domain name, and re-add it, in all lower case.
The various deliverability consultants that have run into this have graciously given permission for me to share this here, so that other folks running into this same issue can more easily figure out how to address it. Thank you all!