iTunes Match turning off occassionally
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.