Error with partial link

I'm not sure if I've used the term "partial link" correctly, or even that there is a correct use for it, but recently I've become annoyed at having to continually do a complete rebuild whenever I make a small, insignificant change. For every time that I make a small change and click 'Build', so that VC 6 recompiles the current file and links it with everything else, there is a time that I make small change and press 'Build' and VC gives me these two errors:

test.obj : error LNK2001: unresolved external symbol "public: bool __thiscall 
core::CEngine::run(void)" (?run@CEngine@core@@QAE_NXZ)
test.obj : error LNK2001: unresolved external symbol "class core::CEngine * 
__cdecl core::createEngine(class video::dimension2d const &,class
 core::IEventReceiver *)" (?createEngine@core@@YAPAVCEngine@1@ABVdimension2d@video@@PAVIEventReceiver@1@@Z)

And doing a complete rebuild fixes these, no problem. Does anyone know why this happens when I'm making changes to an unrelated file? Is it something wrong with the project options? Or is it that I just don't have everything set out properly?