To get games snapshots, manuals, video, etc from ScreenScraper your roms must have the same MD5/CRC or the same filename than records stored as knowledge in the ScrenScraper's database. My experience thinks most newgen roms (like Gamecube) aren't matching very well. That's probly the reason why you get few snapshots for your roms.
By the past, many downloadable roms over the Internet were wrongly dumped. So, it's important to check your romfiles (MD5 + filename) with ClrMamePro to be sure you've the most known/better version file. With the good roms versions you get better scrap's results.
You can find and download XML dictionnaries at Dat-O-Matic and check your romfiles for many hardware with ClrMamePro.
This roms manager tool is also a must have to keep your MAME's roms uptodate. Read it's tutorial carrefully : http://clrmamepro.free.fr/clrmamepro.php
If you're looking to start with great fullsets search "No-Intro 2019" at Archive.org, then browse thoroughly the results page.