PlayerPrefs not saving on Android Phone

I am using PlayerPrefs to save data in my android game.

My problem is when I exit the game and return my progress isn't saved. It used to work now doesn't. The game saves just fine on other Android devices including an identical phone to mine. I have a Galaxy S running 2.2.

Whenever I quit the application the phone vibrates a few times only on my handset. This doesn't happen on the 2nd Galaxy S where the PlayerPrefs works.

Is there a better way to save to the phone, or a work around to this problem?

Thanks