nmake file include problem

Hello:
I have written an app that uses QT and a third party library.
What I am doing is executing the following on my project:

I run qmake -project.
(this generates the .pro file).. I edit this .pro file and update the INCLUDEPATH, QMAKE_LIBDIR and LIBS variables of the .pro files to supply location of the third party library.include/libs folder.
WHen I run nmake after this I get an error saying that a partciular (third party) include file cannot be found / opened. I checked and double checked to make sure that the paths provided to the INCLUDEPATH variable actually exists and so does the file that the compiler complains about. WHat am I doing wrong?
yet I get this error. Any help would be appreciated.

We don't know which file, whether it is a compile or link issue with a missing file (we could guess it is a missing header), or what your pro file and the offending source (if applicable) looks like.

BTW: QMAKE_LIBDIR should not be necessary unless you are doing something quite unusual.