Most likely the vacuum elements are not working. Whether it is a vacuum leak or failed elements you'll either have to find the leak or replace the elements. If you do a search for vacuum element on the parts places websites you'll find the different elements. I'm facing a similar problem on mine. As the vacuum pump eventually shuts off, I've chosen to not worry about the problem.

I disconnected the elements from the vents and opened the centers manually. You'll find one element behind the speedometer and the other behind the glove box. As this gives me center vents I'll worry about the proper solution later.

You can find how to tear apart your dashboard here:

http://kropf.net/w202/Dashboard.html

Brian