Paint() not invoked by QGraphicsScene

Hi,

I'm building a project for school which is partly similar to the DiagramScene-example.
I'm rewriting the arrow class so that it can draw arrows which consist of multiple lines.
My problem is that when I create this item and add it to the scene, the paint() method isn't called by the scene.
I am not allowed to put my code here, but do any of you have an idea on what might cause this problem?

The method signature might not match.