Here are the possible reasons why you have uncategorized transactions in QuickBooks Online:
Transaction is older than 180 days.
The amount in QuickBooks is different than what is cleared on the bank.
The transaction type is different in QuickBooks than at the bank. QuickBooks wants to match a check and an expense with ACH or debit/credit card swipe.
Transaction was recorded in a different bank account.
The transaction from the bank is a payment/deposit of two or more open transactions in QuickBooks.