I didn't see this documented anywhere, but there seems to be a big difference in how you Upload your files. Since I already have my builds set up on Google Drive, I used the "Add External Link" method instead of the Big Red Button "Upload Files".
It seems that external links trigger a browser popup, which has been blocked on most browsers for longer than I can remember. In my case, that meant that 90% of the attempts to get my game failed. Not good. I'll be trying the Big Red Button next and hope I get a better conversion rate.
Not sure why you'd use browser popups, but I would suggest putting a warning somewhere for folks that go that route.