Problems with waveform CDJ-900 Nexus

Sometimes when i'm playing on the CDJ-900 Nexus my waveforms disappear when i'm loading a new track. First the upper waveform disappears, when I load an other track the down waveform disappears too. 

When I restarted the player everything worked fine again. 

I'm still able to play the music when this happens.

Does anybody know how to fix this or what causes this problem?