What conditions would prevent an Invoice from being adjusted by the line?

Hi,

I have a user who had an invoice in which she was able to perform an adjust to bill line.  She wanted to create another adjustment to bill line after the first adjustment was made.  When she goes to select the invoice to adjust, the invoice is no longer available.  The item has not been paid and is still open.

I was wondering at what point and what causes an invoice to no longer be able to adjust.

Thank you.