It could be that you closed your poll too soon before any participants voted and results were captured.
Note that with the Office 365 Add-in, you only need to advance to the next slide- and the poll will open automatically. When you press the 'Enter' button again, it will close the poll and prepare the results before sharing the results in PowerPoint.
If you have closed your poll too quickly, read this article to see how to re-run your poll.