I had this same exact problem.

The problem is likely that you are using qt quick 2, which requires OpenGL. OpenGL does not support the frame buffer used for the small display.

Similar discussion found here:
http://raspberrypi.stackexchange.com...g-qt5-on-pitft
https://forums.adafruit.com/viewtopic.php?f=47&t=57557

A solution would be to use qt quick 1 or qt widgets.