I only have album art in Longplay for less than 1% of my albums. All of the artwork is present both in the Library/Album view of the Music app on my iPhone and my Mac, so that's not the problem. I've force quit Longplay and opened it again, no change. The art isn't changing, either, i.e. none are being updated/added over time; it has X when it opens, and that's it.

I've read the FAQ and as mentioned above, what's discussed there does not seem to be the problem. What else can I look at?