The amount from the bank account is right, but one deposit transaction is missing?

Welcome to the Community, @help-uaefairies-. Please know that only posted transactions will show up in QuickBooks Online (QBO). 

It could be that the transaction is still pending in your bank. Hence, the bank balance in QuickBooks is already updated, but the transaction didn't show up yet. You'll want to check your online banking to verify the transaction. 

For future reference, read through these articles to ensure your transactions are added to your books:

•  Categorise and match online bank transactions in QuickBooks Online
• Reconcile an account in QuickBooks Online

Keep in touch if you need further assistance while working in QBO. I’m here to help.