Don't copy code from the base, run parent::enable_step($old_state);

Also remember that enable/disable/purge functions are designed to be called many times. You should only handle notifications once, not every time this step is called. Best would be to make the enable a step on its own, so you could do something like:
```
switch ($old_state) {
case '' :
//enable notifications
return 'notifications';
break;
default:
return parent::enable_step($old_state);
break;
}
```