Hi there, MWWD.
It sounds like you've just connected your bank account to QuickBooks Online.
Usually, banks only provide transactions for the last 90 days after connecting a bank account. You can download the older ones from your bank website in a supported web connect file format, and then upload them to QuickBooks. Here's an article to guide you through the steps: Manually upload transactions into QuickBooks Online.
You can also check about assigning, categorizing, editing, and adding your downloaded banking transactions.
Please don't hesitate to reach out if you have additional questions about your missing transactions.