Let's perform some troubleshooting steps to download the missing bank transactions in QuickBooks Online (QBO), userlohming_goh.

If you excluded the downloaded transaction, we can undo it. Just go to the Banking or Transactions menu and select Banking. From the Excluded tab, find the transaction you want to recover and tap Undo. Then, go back to the For review tab so you can categorise it.

Just in case you manually recorded the transaction and accidentally deleted it, we can use the audit log. From there we can gather the info and recreate the transaction. Here's how: 

1. Go to the Gear icon and select Audit Log.
2. Select and use the Filter to narrow the results.
3. Tap Apply.

Once you find it, select View in the History column to open the audit history. Then, you can now recreate it by clicking the + New button and select the appropriate transaction type.

We can also get transactions from your bank and manually import them into QBO. To avoid entering duplicates, let's check the date of the oldest existing transaction currently in the account. Here's how:

1. Go to the Accounting menu and select Chart of Accounts.
2. Find the bank you want to upload the transactions into.
3. Tap View register.
4. Find and write down the date of the oldest transaction. Usually, the oldest transaction is the opening balance.

Then, get transactions from your bank by following the steps below:

1. Sign in to your bank or credit card's website.
2. Follow your bank's instructions for how to download CSV files of your transactions to your computer. Every financial institution has different steps. 
3. Check the date range for the download. It should be at least one day before the date of your oldest transaction.
4. Download your transactions. Then, save the file somewhere you can easily find it, like your computer's desktop.

Then, follow the next step in the Manually upload transactions into QuickBooks Online article to finish the process. From there, we can categorize and match online bank transactions into your account. If you need assistance on the steps provided, please contact our Customer Support Team. 

Feel free to visit our Banking page for more insights about managing your bank feeds and reconciling accounts.

You can always update us on the result after contacting our support in the comment below. I'd like to ensure this is resolved for you. You have a good one.