IOS – iPhone 4S using 3G to download songs from iCloud after iOS 5.1 update

Are you logging out of your Apple account?

One possibility is that you are being logged out of your Apple account for some reason, either automatically or manually. If you are logging out of your account manually, then that's probably disabling iTunes Match because the service is attached with your account.

There are several situations in which you will be logged out of your Apple account automatically. For instance, when I first started using iTunes Match, I noticed that for some reason iTunes Match kept turning off. I later realized that it was because I had apps from different accounts installed (such as two accounts from different countries), so when I logged in to a different Apple account from the one that I use for iTunes Match so that I could update some of my apps, iTunes Match would disable itself.

Try clearing the cache

The cache could also be causing problems. I know that it this happens on the desktop version of iTunes; I'm not so sure if the iOS version also has cache problems, or at least I don't know if it's possible to clear any part of the cache on iOS.

To clear iTunes's cache, here are some helpful instructions.

I realise this is months late, but iTunes was never designed to synchronise Libraries in that way. iTunes Match is the closest you will get to automatic synchronisation.

The simplest way to do it is to manually add to each Library.

Even if both Libraries use the same actual source, such as NAS storage, or one of the 2 machines you mention, the Playlist data will not update to reflect the addition of a tune to the other machine - the file would be there, but the player wouldn't realise.

It is possible to manually transfer the playlist from one machine to another in such a case, but it needs re-syncing every so often, it can't be automated. The file-paths are absolute rather than relative, so this requires some patience & a lot of find/replace, then must be re-imported to the other machine. For your scenario I don't think that would be the way to go, but I can add instructions to this answer if you need.