I could only reproduce the problem with sizes if 320×480, 360×640 for the GameWindow.
The reason for this is that the ParallaxScrollingBackground item uses MultiresolutionImages internally. Which leads to the answer given in this thread: https://v-play.net/developers/forums/t/multiresolutionimage-images-not-showing-on-smaller-screens/
What is the size of your Scene? Could this be the cause of your problems?