I cant understand how my one income transaction is not able to link to the associated invoice?

I'm here to provide details about linking transactions in QuickBooks Online (QBO), sian4.

QuickBooks analyses your income transactions against your data (invoice) entered in the system to link/match them. They should have the same information such as the date, customer name/payee, type, and amount.

Here are some factors you're unable to link one income transaction to the associated invoice:

• It has been reconciled.
• It was entered using a different bank account.
• It has been previously matched to another bank downloaded transaction.
• It's outside the date range displayed.

After checking every aspect provided above and everything is all set, you can now match the income transaction to the associated invoice. Let me guide you how.

1. Go to the Banking menu.
2. If QuickBooks found a possible match for the bank transaction, it displays an outlined green box under the Category or Match column says 1 record found message.
3. Click to expand the transaction, then review its details.
4. If it is the correct match, select Match under the Action column. This will move the transaction from the For Review to the In QuickBooks tab.
5. If it's not the correct match, select the transaction row then choose to Find Other Records, Add or Transfer the transaction.

In addition, I'd recommend reconciling your accounts every month. This is to manage the accuracy of your transactions and monitor the growth of your business. You can refer to this article for the complete details: Reconcile an account in QuickBooks Online. 

Please let me know if you have other concerns. I'm just around to help.