At least three times over the past few weeks, iTunes Match has spontaneously reset itself on my Mac. By this, I mean that the iTunes Match item reappeared in the Store section listing. Selecting the item revealed that iTunes was again proceeding through the three steps of matching all of my music (Step 1: Gathering information about your iTunes library).
I have no idea what precipitates these resets. It doesn't appear to be due to anything I have done. I suspect some hiccup occurred on Apple's servers.
While the rematching remains mysterious and irritating (as it can take hours for the process to complete), it gets worse. Most often, the rematching never completes. Either it simply hangs, with no message and a progress bar that never moves, or an "unknown error" message pops up, suggesting that you try again later.
When this happened to me, after retrying three or four times, I have success: the matching completes and I am back in business. However, others have not been as fortunate. A search of Apple's Support Communities reveals () numerous users with the same symptom that remain stuck no matter how many times they "retry."
In such cases, the most-likely-to-be-successful fix has been to select the "Turn Off iTunes Match" item from the iTunes' Store menu--while holding down the Option key. Supposedly, this forces deletion of old iTunes Match cache data which can be the source of the problem. [Note: There is some confusion, which I have yet to sort out, as to whether to hold down the Option or the Shift key. I have seen recommendations for both. In either case, the menu item text does not change when holding down the modifier keys, so there is no sure indication that anything special is happening. You have to cross your fingers and hope.]
After this turning off of iTunes Match, the next step is to turn it back on again. With a bit of luck, the matching process should now complete successfully. That's what most people have found. For a minority, the matching continues to fail. If so, grab a cup of coffee and wade through a long on this matter. Just about every possible cause and fix is mentioned somewhere within it. Hopefully, you'll find a nugget that will do the trick.
When you create a new playlist, especially if you create it on an iOS device, you may find that the list gets duplicated--sometimes thousands of times (as described in this ). This seems especially likely if you have Airplane Mode enabled on your iOS device at the time. The duplicates are usually empty, but they still take up a lot of listing space and are obviously annoying.
The symptom appears linked to iTunes Match. In fact, in some cases, playlist duplications occur shortly after turning iTunes Match on, even if you don't create a new playlist. In any case, once the duplications pop up, it's likely that they will spread to all your devices that have iTunes Match enabled.
The first step in resolving this problem is to get rid of the duplicate playlists. This can be a time-consuming pain to do "manually" deleting playlists one-by-one. To speed things up, you can launch Automator on your Mac and select the Remove Empty Playlists action from its Music Library. Alternatively, Doug Adams has that will do pretty much the same thing. Even simpler, you can purchase Mr. Adams' utility ($1.99 in the Mac App Store) and delete empty playlists with just a click.
If your duplicates are not empty, you can create an AppleScript that will delete all copies of a playlist of any name you specify. An example script for doing this is posted in the above-linked Support Communities thread.
Next, to hopefully prevent this from happening again and to bring back any non-duplicate playlists that may have disappeared at some point, select "Update iTunes Match" from the Store menu in iTunes. If this doesn't work, try the same fix noted in the previous section: Turn off iTunes Match, while holding down the Shift or Option key, and then turn it back on again. If you do this, Apple recommends that you first disable iTunes Match on all of your iOS devices and then re-enable it on these devices after re-enabling Match on your Mac.
If none of this works, check the above-cited thread for further variations on these suggested fixes.