Dear @digischool_digitalen, as many other components, like BT comm's, or global variables, cease to work when switching between screens. Please consider that switching between screens is like running a completely different app. To avoid this we often suggest to use "virtual screens" instead (if you dig the forum with such tag you'll find many similar topics).
Moreover, there is a FAQ section (thanks to @ABG) that teaches how to correctly switch between screens.
Best wishes.