Have you ever faced the scenario of notification subscription which became disabled automatically every half hour (approximately) ? did you enabled it  and again after half hour it became disabled again ?

So.. why does it happen ? – OpsMgr has mechanism which validating the owner of the notification subscription which means that if the user who created the notification subscription is disabled or not exists anymore in the Active Directory organization the OpsMgr’s mechanism will disable the notification subscription.

To fix this problem you can :

1. Recreate  the notification subscription with working user. or –
2. expoert the Microsoft.SystemCenter.Notifications.Internal, locate the user’s SID and put new SID of existed user. after done, import the new edited XML.

I think you should create the  notification subscriptions with the user who runs your OpsMgr services, probably it never wont be disabled or deleted from Active Directory.