CDJ 2000 NEXUS 2 MASTER light doesn't shine

I have recently bought a pair of CDJ 2000 NEXUS 2. Once I turn on them in one of the CDJ, MASTER KEY light doesn't work (doesn't shine when I press it) but key works perfectly because when I press it, I can see it on the screen but key doesn't shine as SYNC key for example, when it is pressed. I don't know if it is a hardware problem or a firmware problem. I would thank any help because those CDJ are still on guarantee and I don't know if this problem can be fixed or I have to claim to MUSICSTORE (place where I bought them) to bring it back.

Thanks a lot!!!