What is a diplomatic way to tell a professor that progress hasn't been made because of their lack of communication?
What is a diplomatic way to tell a professor that progress hasn't been made because of their lack of communication?


You don't. 

That is, I don't think that would be very productive, diplomatic or not. I wouldn't focus on their lack of communication in the next meeting you have with them at all; I would instead focus on using that next meeting you have with the professor to come to some resolution on the things that are delaying progress on the paper/project.

As you mentioned in the comments:


  I essentially have done everything I can, within reason. I've selected and done background on directions for the paper.


So, this is what you have, and this is what you show them. Additionally, any questions about the scope/direction of the paper, etc. should also be brought up during the face-to-face meeting. 

To address additional information posted by OP:


  But I strongly suspect (and he seemed to say) that he expects me to have done more. For this question, assume that the professor will ask why more hasn't been done on the paper, so the subject will come up.


We all have limitations. You were in charge of certain aspects of the paper/project, but you were not able to achieve a certain level of progress that was set. Just be upfront about that. Specifically, tell the professor what it is you are stuck on and ask them during your face-to-face meeting if they have any feedback to offer.