DJM 400 - Sometimes when mixing, the sound is cut completely on one side...?

Hi everyone!

I am totally new to Dj'ing so please forgive my lack of technical terms.

I have a 2 x CDJ400 + DJM 400 set up.

When doing a mix, sometimes the sound is cut completely from one ear of the headphones.  But the headphones are not the issue here as they work in other systems perfectly.

I record using Audacity, and when I playback recordings, the sound can suddenly just come from one speaker (which is at the same point of when I hear from only one headphone ear in the mix).

I thought it may be the track itself, but I have used the same tracks in different mixes, and there isn't any consistency as sometimes this happens with a track, but next time it doesn't, which rules out it being a track problem.

I can only think it's to do with the DJM 400.  Has anyone ever come across such thing? Or has any suggestions?

If you need more info let me know; as I say I am new to this :)

Thanks for any help! :)