Cannot find include file

Cannot find include file

I have a solution with 2 projects in it:
ControlGUI
FunctionLIB
In the FunctionLIB I use two APIs for external hardware, called SDK1.h and
SDK2.h
The lib is linked by the GUI, but the GUI only includes the FunctionLIB.h
When building the lib, the headers are found since the include directories
are set properly. But when compiling the GUI, which uses the LIB
functions, I get include files not found. I have to add the include
directories to the GUI directories as well.
Why is that so? The LIB finds it just fine, but the GUI indirectly throws
an error, and I do not understand why it would need those includes in the
first place.