Sync working fine but some older bank transactions are missing and they are not getting picked up by QB

Did you just converting data from QB Desktop?

Thank you for joining the Community, @amybanala.

I can share a few insights on how to get your missing bank transactions into QuickBooks Online (QBO).

Your bank may be unable to sync the entries between September 2021 and June 2022. To load them (October 1, 2021, to May 31, 2022 transactions) into the system, you'll have to download those transactions from your bank and import them to QBO using the WebConnect feature.

In addition, make sure that the file is in the correct format (commonly CSV file) before you can upload them. The maximum file size is 350 KB. You may have to shorten the date range to reduce the file size.

Here are the steps to upload the file:

1. In QuickBooks Online, go to Transactions, then select Banking.
2. Select the blue tile for the account you want to upload the transactions into.
3. Select the drop-down ▼ next to Link account and then choose Upload from file.
4. Choose Drag or drop or select files and then select the file you downloaded from your bank. Then select Continue.
5. In the QuickBooks account ▼ dropdown, select the account you want to upload the transactions into. Then select Continue.
6. Follow the onscreen steps to map the columns on the file with the correct fields in QuickBooks. Then hit Continue.
7. Pick the transactions you would like to import. Then select Continue.
8. Select Yes.
9. When you accept your transactions, select Done.

For more details about the process, we can check out this article: Manually upload transactions into QuickBooks Online.

In addition, I'd like you to review this article in case you need help about editing the file: Format CSV files in Excel to get bank transactions into QuickBooks.

You're more than welcome to post here again if you have questions regarding your entries or data in QuickBooks Online. I'm here to help.