It’s not the App.
Just as important as clean optics is that the material has to be flat against the crumbtray.
Different batches of any materials can be slightly warped, especially if your live in an area subject to humidity swings.
Printing a set of holddown pins will really help with this. Once secured to the tray, you should be able to tap all over the material and hear a muted sound. If an area is louder than the others, it’s not flat.