This is a known Traktor bug.
When adding new tracks to Traktor‘s collection, it does not detect the track as being new and forgets to load the artwork or calculate the waveform for the new track.
To fix this, edit the track displaying the problem and then just click ‘Ok’. That forces Traktor to reload the cover art. If you’re missing the waveform data, analyze the track with only ‘Gain’ (under Special) selected and that will re-create the waveform data.
Another suggestion is to force Traktor to reload the data by running a ‘Consistency check’ on the Traktor database (right-click on Track Collection and select ‘Check Consistency‘).