If you haven't done so already, I'd say it's time to call Quicken Support on the phone and request help. Let them do a screen share session with you to review a download log file on your computer, to determine the nature of the problem and to figure out how to fix it.
Quicken Support:
Hello @Tim K
Thank you for reaching out on the community and telling us about your issue. I do apologize for the issue you're having. Are you having issues with all of your accounts or some of your accounts. What error message are you receiving? What FI are you having issues with as well.
With more information we'll be able to take a look to see what exactly we can do in order to fix the issue you're having. Let us know once you get the chance.
Thanks,
Quicken Francisco