Painting issues with QT5

Hello, While porting my app in Qt5, i have several problems with the call of the method paint in QGraphicsItem by example. Example : i have a QGraphicsItem which works as a button. I catch the signal pressed() , i change the button’s color but the method paint is not called (i tried to force it by calling update() but it doesnt work too). So the new color never prints. Then i catch the signal released() , I set the oldest color. So in my loop, the button’s color never changes. Any idea how to solve this problem? I hadn’t this problem with Qt 4.6 but i setted the graphicssystem as raster and it isnt possible anymore in QT5. Thanks.