Dynamic Header generation

Hi EveryOne,
I have a problem with header files.
We are working on a Huge project with over 900 MB of Source code.
The Source is compiled in units with each unit generating header files for the next unit. These header files comtain #define directives without which the next unit would not compile.

When the unit_1 is built in debug mode it generates all the necessary header files.. but in release mode it is not generating all the header files.

We are using VC 6.0 with out any service packs or what ever...

Where are the directives to generate these header files. Where should I be looking for it. Any help or information is welcome.

Best Regards,
Vijay