Indesign: Lines appear systematically in the background

Hello,

I am a relative newbie using Adobe Indesign and have been hitting a seemingly easy issue for hours, without managing to solve it -- I hope you can help me.

I am using the Line tool to trace a simple set of lines. I am only using one layer for the whole graphic. For some reason, the line appears systematically in the background, even that it's clearly at the top of the layer.

Here is a screen shot, showing 4 lines. The background is transparent, which means that the black line appears as if it is on top, but it's actually below. The white line is invisible, and the yellow and green line are faded, because they are behind the background image.

I am only using 1 layer, and these 4 lines are clearly shown at the top:

If I try to right-click "arrange", I can only send the lines "backward", but not to the front, since they are already at the very front.

Could anyone help me solve this mystery? How do I get the lines produced with the line tool to appear on top?

Thanks a lot!!!