Xkimo,
I just did a complete re-install for my Series (not doing the movies again, took ages to get them up to scratch adding metadata) and allowed fresh metadata updates for all series. First time around, I had the same issue, couldn't really get much metadata. The thing appears to be that it grabs metadata in the background, but if for any reason, you close the application, it seems to assume it's done (not sure if this is a bug or not JL). Basically, to get it to download ALL application data, I browsed my series folder and sat it there for quite some time. I didn't follow any links to series, just left it at the main page. When I came back to the screen, all metadata had updated and all images were in place.
To test this, delete all metadata folders and browse to your series, and do as I did. Leave it while you have dinner or something and come back to it.
Side Note:
It's new series time, and I just added the first episode of Season 5 of Entourage, and thought I'd allow it to test the waters when updating a new season. It grabbed the thumbnail fine, but still only appears to have grabbed the small amount of data from the provider cache folder, didn't refresh the provider cache data. I don't mind removing the Ep from the folder and trying anything you need to test this out, but yeah, thought I'd pass this on (not hard to fix, just remove the .metadata and let it refresh, but thought it would be worth testing). That being said, I'm sure I'll have other new seasons in the coming weeks, so can keep testing it with them.
Cheers,
Schlep. :D