Soundwave not appears as a line in rekordbox

I was adding some new songs to rekordbox but the soundwave in the preview windows and in the deck window appear as a white line. Other songs have only some part of the sound wave at the beginning and then the white line appears for the rest of it.

Please find a picture where the problem can be observed highlighted with a red circle.

The soundwave is correctly displayed in the horizontal/vertical view and the audio is not being affected. I wonder why this is happening.

Thanks