Album Art not appearing for certain songs

[Subject95]

I have downloaded a mass amount of songs but some of them lose their album arts, but at the same time, albums load up very slow and for albums i've never looked at before, the album art doesn't load up. And I really like the album art so can anyone try and help with both situations? I have also tried with the desktop app and the album art loads up fine so i'm not quite sure what the problem could be.