@spinningvynal > The obvious question is does the mixer channel for which that CDJ's audio is connected match the player number display on the CDJ itself? For example, it you are connected to CH4 on the mixer, the player number must also be 4. If it were connected to CH3, it needs to be set to 3.
To manually assign the player number, remove any USB or SD devices and disconnect any USB or ethernet (LINK) cables, then go into the player settings to assign the number to match the mixer channel.