-Silver- Just guessing here, but if you're destroying the object, and then setting isActive to 0, you're not actually applying the variable to anything (because the object doesn't exist). So the only object that will have isActive = 0 is the one you've initially placed in your layout until it becomes active.
Not sure if this would work for you, but if you only ever want to have one instance of an object on screen at a time, could you not just keep reusing the same object? So instead of destroying the object, set isActive to 0 when it moves off the left side of the screen. Then when it's time to use it again, just move it to the right side?
Not sure if any of that makes any sense in your situation, though ;)