iTunes Match not downloading high bitrate version of songs
In addition to iTunes Match not working properly, I also noticed that I was unable to add new songs to my iTunes library. After searching online, I came across this thread, in which some of the posts recommend logging out and back in to your iTunes Store account. So, I did that, and both the iTunes Match problem and the add songs problem were fixed.

Although for some reason, unchecked songs still aren't downloading higher bitrate versions, so I haven't completely figured things out yet.