Did you select the 'Midi clock in input' in the audio & midi settings of Spark?
You should select the traktor midi clock channel.
If 'clock follows stop/continue' is on, Spark should start/stop whenever you start/stop Traktor, and stay synced while playing.
If 'clock follows stop/continue' is off, Spark won't start or stop when Traktor starts/stops,
but will stay in synced, if Traktor is playing and you play start manually in Spark.
Let us know,