Thank you very much for helping me to check for button click signals. The problem was that! My custom button was not emitting the signal due to a small mistake in void mousePressEvent(QMouseEvent * event).
Qt Code: 
  1. void mousePressEvent(QMouseEvent * event)
  2. {
  3.     if(event->button() == Qt::RightButton)
  4.     {
  5.         return;
  6.     }
  7.     else if(event->button() == Qt::MiddleButton)
  8.     {
  9.         return;
  10.     }
  11.     else    // I forgot to add the else case and return the necessary event
  12.     {
  13.         QToolButton::mousePressEvent(event);
  14.     }
  15. }
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Thank you.