I had this same problem. It was driving me crazy and I finally figured it out. On my main computer, I keep an actual file for of all my music. None of it lives solely in the cloud. However, on all my other iTunes Match devices, I stream all my music from the cloud (which includes songs I've purchased from iTunes as well as ones I've imported myself). One song in particular was stuck in limbo with the exact message you wrote. This particular song is not available in the iTunes Music Store. I didn't want to delete the file and reimport it because I like keeping all my play count and date added info. My other devices linked to my iTunes Match account knew that the song existed, but it could never actually play this particular song. I'd always get an error message.
Here's the solution: On another computer that's linked to your iTunes Match, delete the song. It will only ask you if you want to delete it from iCloud as the song file doesn't actually live on that computer. If you say yes, on the original computer where the song file lives, you'll now see the song file has a cloud icon with the X in it. From there, you can control-click on the song file and select, Add to iCloud. Then, it will add it to iCloud and will no longer be stuck "processing."