JNI: dll function works ok in C++ main, but not with dll wrapper

Question

I have an a.dll (not modifiable as i do not have the source) with a function bool openPort(DWORD mem).
I wrote a c++ main, loaded this dll using LoadLibrary, and the function works well.
It returns true.

Now, I need to call this function from Java via JNI.
I wrote another b.dll with a function like so

JNIEXPORT void JNICALL Java_MyClass_openPortFunc
(JNIEnv *env, jobject obj, jint pMemPhy)
{
    hInstLibrary = LoadLibrary("a.dll");
    typedef bool (*openPort)(DWORD);
    openPort _openPort;
    _openPort = (openPort)GetProcAddress(hInstLibrary, "openPort");

    DWORD memAddr = 0xda000;
    if(_openPort(memAddr)){
        cout << "ok" << endl;
    }else{
        cout << "failed " << endl;
    }
}

This however, causes the openPort to return false despite using the same parameters.

I hope someone can advise me.

Thank you. :)