Okay, I'm new in the biz and need some advice in dealing with a delicate situation. The client for a 37 minute family documentary project liked the rough cut and made some suggestions to include in a "final" cut which was delivered to the client before Christmas. After the holidays, the client comes back and said her family had some more "suggestions" to add photos and cut out some portraits they didn't like, even though the primary client approved of them months ago. Add to this the fact that the client has only paid a third of the bill so far, and is overdue on the next installments. I feel that the client will pay up, but is using the balance due as a sort of carrot to get more and more things added. Does this happen often? should I feel reluctant to keep putting hours in without another payday? Unfortunately, the only specification for final payment was when the final project was delivered...but I feel like this could go on forever.