Placement of extern variable

Question

So I declared 4 extern variables just under the list of headers in my main.

I defined the 4 variables in a click event listener function in my GUI source code. The code compiled.

Now reorganizing the event listener function, i decided to put some of the code into another function, which is called by the event listener function. It won't compile and the compiler says that the 4 variables are undeclared identifiers.

I was under the impression that global variables should be used everywhere... if so, why is that not the case for this particular case?

// 4 extern variables
extern QXlsx::Document BreastCancer;
extern QXlsx::CellRange Range;
extern QXlsx::CellReference BottomLeft;
extern std::vector<patient_data> patients;

int main(int argc, char *argv[])
{

    QApplication a(argc, argv);

    Neutropenia_Main w;
    w.show();

    return a.exec();
}


//declaration
void Neutropenia_Main::on_Load_Patient_Data_clicked()

{

QXlsx::Document BreastCancer("Breast_Cancer.xlsx");
QXlsx::CellRange Range = BreastCancer.dimension();
QXlsx::CellReference BottomLeft = Range.bottomLeft();
int bottomrow = BottomLeft.row();
std::vector<patient_data> patients(bottomrow-1);

extractfromxlsx(bottomrow); // function call

void extractfromxlsx(int bottomrow ){
    patient_data temppatient;
    QVariant tempdata;
    for (int i  = 2; i < bottomrow; i++)
        {
            temppatient.setCaseNumber(BreastCancer.read(i,1));// compiler says BreastCancer is undefined

            tempdata = BreastCancer.read(i,2);
            temppatient.setGender(tempdata.toInt());