If you're unable to download your bank transactions in QuickBooks Online, It'd be best to try updating your bank manually to refresh the connection between QuickBooks and your financial institution.
Here's how:
Go to the Banking tab.
Select the blue tile at the top for the bank account you want to update.
Select the Update button. If you see a message about additional authentication, follow the on-screen instructions. Not all banks require this, but some do for added peace of mind.
If you still can’t download new bank transactions, I suggest using a private browser to help us determine if this is a browser-related issue. You can use these shortcut keys:
Google Chrome, press CTRL + Shift +
Mozilla Firefox: press CTRL + Shift + P
Safari: press Command + Shift + N
If you're able to connect your bank account using a private browser, you may need to clear the cache of the regular browser you're using. Unusual behavior happens if your web browser has accumulated cache as files are being saved to the hard drive.
You can also sign in to your bank’s website from QuickBooks to check if the issue has something to do with your bank.
I’ve added articles on what to do if bank transactions won’t download and if you encounter an online banking error: