DJM900 NX2 - Digital Line input Issue

I have been runing this mixer with the CDJ200NX2 since they came out many years ago. They have always been flightcased, cabled up and hardly ever touched. 

I noticed the volume on channel 2 was slightly lower than that on channel one. I played the same track via both CDJ's with the bass/mid/treblle/gain all the same and channel 2 was lower on the meters. 

 I thought this could be a cable issue so i unplugged the digital input for channel 2 and placed this in channel 3 and then channel 4 and it was then the same as channel 1 on the signal strength. 

I plugged it back into channel 2 and noticed a dip in the sound level. I have no idea why it has suddenly done this, they are not hammered and used for Mobile Dj work in a coffin style case.  Any ideas what the fault could be? I havent tried the RCA inputs as yet and the digital cable seems fine if it works ok on the other 3 channels.