I'm here to ensure that your bank transactions will be downloaded successfully in QuickBooks Online. However, can you tell me the name of your financial institution that you're working with? This helps me check if there's a reported issue related to your concern.
For now, we can try updating your bank account manually. This process refreshes the connectivity between your bank's website into QuickBooks and helps download recent transactions.
To do that:
Click Banking on the left panel.
Select the account from Bank and Credit Cards drop-down menu.
Press on the Update button.
If doing this doesn't make a difference, I recommend reaching out to your financial institution to verify if they don't have any ongoing system maintenance that may lead to this issue.
For now, to get your work done, you may manually upload your bank transactions in the system. This way, you'll be able to categorize and match them smoothly.