When downloading data to QuickBooks, make sure you have followed the supported format to get your transactions in.
Let's perform some troubleshooting steps to isolate the issue. To start, let's access your QuickBooks Online account using a private browser. There are times that cache and cookies stored on the browser cause problems when running websites.
These shortcut keys you can use:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox and Microsoft Edge: press Ctrl + Shift + P
Safari: press Command + Shift + N
If this works, please go back to your regular browser and clear the cache. If not, you can use another supported browser. It could be that the one you're currently using has a temporary issue with QuickBooks.