Thanks for posting in the Community! In QuickBooks Online, there are several reasons why you're unable to match your transactions. These are due to the following:
Transactions have been previously matched to another downloaded transaction.
Transactions were not entered using the same bank account.
The transaction entered manually in QuickBooks could have a different number, amount, date or payee from the downloaded expenses.
Transactions are already reconciled.
You may check the transaction you're trying to match to the bank feed to see if it is not matched or reconciled yet and if it is a payment, make sure the the bank account you've selected in Deposit to field is the same as with your bank feed.