Property sheets in Visual Studio basically form a stack, with the higher sheets inheriting properties from lower sheets as specified for each property. For most properties this is a simple override-or-inherit choice, but for include paths it is slightly more complex because include paths can still be added by a sheet even if property inheritance is enabled (i.e. the set of paths is merged in this case).
Failure to enable inheritance (on include paths) will suppress all earlier paths, leaving only the paths in the current sheet. To enable inheritance, go to the property manager, find the appropriate property sheet, open the page with the include paths, select "edit" from the dropdown box, and finally enable the checkbox marked "inherit from parent or project default".