Songs missing in iTunes

Hello. I have a problem. Actually I have many problems with Apple Music, but I'm hoping they'll fix these things soon, but here's one which is very annoying. I have added some albums from Apple Music to my library (downloaded the tracks to my computer). I also have them on my iPhone. But, recently I noticed that I have only few of the songs from the album on my computer, the rest just disappeared. When I go back and find that album in Apple Music, I try to add the whole album to my library, or download individual tracks, but it doesn't work. Clicking on the plus button literally doesn't do anything. Can anyone suggest what to do? Thanks!