After I updated rekordbox to ver. 6, [?] is displayed on a track list.

In rekordbox ver. 6, you can choose the new style of waveform [3Band]. However, tracks analyzed by the older version of rekordbox don't have the 3Band data, so the [?] icon appears.

Tracks with the [?] icon can't show the 3Band waveform even if you select [3Band] in [Preferences] > [Display] > [Waveform] > [Colors].

If you export one of these tracks, you won't be able to view the 3Band waveform in [WAVEFORM COLOR] on the CDJ-3000, even if you select [3Band].

To fix the problem, right-click the track and select [Add New Analysis Data] from the context menu.