DDJ FLX4 - Crossfader not Fading either Channel

Hello

 ive just started with the FLX4 using Serato DJ Pro

How can I get the crossfader to work?

i.e. Both channels are playing tracks that can be heard no matter what position the crossfader is in

The crossfader is not cross fading anything.... like it doesn't work

Im sure i'm missing something

Can you help?

Thank you