Black screen using WebGL

i built an AR Projekt using WebGL.
iOS everything is okay. Android mostly. But on some Devices i have a black screen. I don’t see a camera feed nor the Szene. If i present the marker, i hear the audio. So the tracking works and the scene seems to load korrektly. But i just have got a black screen. Not on every Android Device :frowning_face_with_open_mouth: Some are running fine.

What can i do?? :frowning_face_with_open_mouth: