Hi simon-wakeford!
Thanks for reaching out to us. Let me help you in importing the bank transactions.
When you manually upload transactions, you'll want to ensure that the date format in the file matches with QuickBooks Online when you do the mapping. This could be the reason why you received an alert. In addition to that, make sure that the fields match the columns of your file so that the system will be able to scan the right data.
You can check these articles. These further explain how to format the file and fix manual import issues:
Once done, you can now categorise and match the downloaded transaction.
Leave a comment below if you have other concerns. Take care!