When transactions are missing, you can simply update your account to upload the data.
Here's how to do it:
Go to the Banking and select the Banking tab.
Click the Update button located in the upper right.
Once done, look for the missing transactions. If there are no new transactions added, then you can manually upload them using the CSV or Web Connect. This is a way of downloading your transactions directly from your bank's website and import them into QuickBooks Online.