How do I bill addition withheld retention amount after total withheld was previously billed?

We set up a project with retention being withheld. The billing terms were set with the billing method as 'Total Withheld Amount'.

We previously billed the total that was withheld.

The project continued to bill and continued to withhold 10% for retention. We now want to bill the additional amount, but are unable to do so. The 'Add' button under billing method is grayed out.

Do we need to change the billing method?

I am new to working projects and have been researching how to do this, but can't find an answer. We really need to get the additional withheld billed out to the customer. Any help is greatly appreciated.