The original CDJ-2000 players do not have Sync. The beat grid feature in Rekordbox was initially developed only to give quantized loops and quantized FX with compatible mixers (i.e. DJM-2000, DJM-900 Nexus) and the RMX-1000. The automatic analysis in Rekordbox does not always get the beat grids perfectly aligned every time, so that's why there are some buttons for moving the blue grid markers and stretching/shrinking the space between the markers. You will need to take a few moments when you load new tracks into Rekordbox to check the beat grids and make some minor grid adjustments.
The new CDJ-2000 Nexus players do have a Sync feature, however. So if you are looking for auto beat sync then you'll need to buy the latest players which were just released this month.