Can I use Grapploct's "Octolock" attack to a new Defending Pokemon if the previous one switches out?
If you use the attack again, yes. You just can't apply more than one Octolock effect to the same Defending Pokemon.




x2
OctolockUntil this Grapploct leaves the Active Spot, the Defending Pokémon's attacks cost 
more, and the Defending Pokémon can't retreat. This effect can't be applied more than once.


Tough SwingThis attack’s damage isn’t affected by Resistance.
A body made up of nothing but muscle makes the grappling moves this Pokémon performs with its tentacles tremendously powerful.
Can I use Grapploct's "Octolock" attack to a new Defending Pokemon if the previous one switches out?
If you use the attack again, yes. You just can't apply more than one Octolock effect to the same Defending Pokemon.
Grapploct's "Octolock" attack says its effect can't be applied more than once. What does that mean?
It means that if the Defending Pokemon is already under the effect of Octolock you cannot do it again for an additional 
cost. In other words, it does not stack.