Walk-back test not working

I tried to tune my button settings with a walk-back test according to Berger, but the arrows positions have not been measured. But if I shoot on a traget face it works. Is this a bug?

This looks like as if you were using a black target and just pinned the blue cross, right? That means that all arrows that landed on the black underground could not be read by RyngDyng. To avoid this problem, use a sheet of paper as a background, such as a flipped 122 target face (it can be a rather used one for that purpose).

The same applies if you are using a completely blank and black target for training. It might work without paper when light conditions are very good and the target is rather gray than black and the arrows are really black. Or, if the arrow shafts aren’t black. RyngDyng just needs some visible contrast between arrow and background.