Discogs metadata retrieval doesn't work with mixed tracks/discs. Take for example this release: https://www.discogs.com/Various-Hed-Kandi-The-Mix-2006-A-Deliciously-Stylish-2CD-Mix-Featuring-The-Finest-Moments-From-Kandi/release/575970
If I enter 575970 for the Discogs Release ID, Mp3tag only retrieves metadata for the 10 tracks on CD3 which is the non-mixed disc. The other two discs are mixed and are not being retrieved.
The songs of CD1 and CD2 are entered as subsongs (which is probably not intended and can probably corrected on Discogs). Mp3tag currently doesn't import subsongs, see
for a related bug report.
Thanks for the reply 
CD1 and CD2 are mixed. Though they are separate songs on the disc, they are played gapless. So, I think there's something to say for both ways of inputting the data on Discogs (with or without 'subsongs'). But I'm not an expert on Discogs metadata by any means. I'm sure there's something about that in their documentation; I'll take a look.
In any case, is there a reason Mp3tag doesn't import those 'subsongs'? Is it a technical limitation (api?), or just something that's not been implemented?