Searching for album art using uris

When I have a (long) list of spotify uris in order to get the cover art of an album, I get a (long) list of empty results back. However, if I use the same API call, but only a list with one or two entries, I get the cover art back for those two entries. (even though the uris are the same in the long list and the short list). Is there a reason for this behaviour? Is it better to send many requests with one uri instead of a single request with many uris?