GetProcAddress not returning address

I have this code in my dll:

Code:

---

//mydll.cpp

extern "C"
__declspec(dllexport) LRESULT CALLBACK CallWndProc(int code, WPARAM wParam, LPARAM lParam)
{
        //some code

        return ::CallNextHookEx(hHook, code, wParam, lParam);
}

---

When I call:
HMODULE hMod = LoadLibraryA(dll_file);
FARPROC procAddr = GetProcAddress(hMod, "CallWndProc");

It doesn't return the address (I get 0x00000000). But when I remove the keyword "CALLBACK" like so:

Code:

---

extern "C"
__declspec(dllexport) LRESULT CallWndProc(int code, WPARAM wParam, LPARAM lParam)
{
        //some code

        return ::CallNextHookEx(hHook, code, wParam, lParam);
}

---

Then it returns the address. Why is this?
Note: When I created my dll, I didn't use a header file (ie. mydll.h)

Edit: I also want to point out that I am able to call the functions from mydll (in C# using dllimport) regardless of whether I add or remove the keyword "CALLBACK" (OR "WINAPI"), which is bacially __stdcall.