Headphone Cue 1&2 not working, Master working (DDJ-RB)

Hey people,

I have a little problem on my DDJ-RB: The Headphone section is not working right. The volume and master of the Headphone section is working, the Cues are not. So I cannot put only deck 1 or only deck 2 on my headphones. This makes mixing quite impossible for me. As the master is working on the headphones, I don't believe it is a problem of the settings.
I hope you can help me out, thanks in advance :)