The class name for the managed objects you are creating wants to correspond to the names of the classes to be created - so I'm not sure what you mean by you reset them to default, but say your prefix is AB, and your class is Thing, then you'd want the class attribute to be ABThing. If you had already done this I may have misunderstood what you meant by returning them to default. Also, it's worth noting that if you start your application again, after changing the Core Data Model, then you have to delete the current core data model - you can use:
NSError* error = nil;
[[NSFileManager defaultManager] removeItemAtURL:<URL For your Store> error:&error];