The reason the payment is unable to 'knock off' is due to the payment has already been knocked off. Once you reverse the payment, you will able to see the payment in the knock off list again.
To reverse the payment:
Go to Transaction, Reverse Knock Off.
Select the Unit No.
Click on the payment that needs to be reversed.
Click Apply.
Select the Invoice.
Click Reverse.
Click OK.
Alternative Method for Reversing Knock-Offs
If the above steps do not resolve the issue, you can try the following alternative method:
Navigate to the Owner Ledger.
Search for the relevant record using the Unit No..
Locate and select the Invoice Document No..
Click on Document Details.
Access Payment Details and note the Payment Receipt No..
Go to Transactions > Payment and search using the noted Receipt No..
Click Reverse and select the corresponding invoice document to complete the reversal.
Note: This also can be caused due to the knock off listing as they have not done the full knock off. This happens when there is payment done, but no invoice attached.