Most likely, you have created either a large or “heavy” presentation. The solution: Press Refresh first. Try again. If the problem persists then:
Go to Export and open the All cards tab.
Select Custom and set a range like 1-15 to export only these cards.
Repeat with the next range (e.g. 16-32) and rename each file immediately (Part 1, Part 2, etc.).
In PowerPoint, merge the downloaded parts into one presentation.