Let us know how they are doing.. One of the things many programmers and detail guys don't do very well at is the customer service side of things.. So in cases like this I always try to assume it's all good, they just don't want to show something not quite ready yet.. That's why when William and I JV on a project I tend to handle the CS side of things and let him keep his head down and code..
And their scheduling issues really aren't your problem when it goes on for months.. Definitely another case of smaller milestones being a good idea..
1. payment to start
2. payment for layout and framework
3. payment for functionality
4. payment for design
5. payment for done
are the general steps.. They can be smaller as the complexity goes up..