When the motorised platters are turned on (or in use), the Serato DJ Smart Sync feature does not work correctly. This is because of the variable speeds of the Numark motorized platters which cause the fluctuation of the track BPM. 

As a work around we suggest using the ‘simple sync’ option within the Serato DJ Setup screen under the DJ Preferences tab. Alternatively you can also turn off the motorized platters on your Numark hardware, which will enable you to use the Smart Sync option.