I had the same problem (media not downloading for some known Genesis ROMs) and figured out how to fix it.
Go to LaunchBox game DB site and find the game. Copy it's title.
Go to metadata editor (select game and press CTRL+E), then paste copied title instead of the title that is filled in (you'll notice it's slightly different), select relevant variant from the dropdown, press button to the right of the textbox ("Update metadata" or something like that).
Go to media tab, now you can download images.
