Attacks » Octolock
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.
Source: Sword & Shield FAQ (2020-01-24), TPCi Rules Team (2020-02-06)
Attacks » Octolock
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 {C}{C} cost. In other words, it does not stack.
Source: Sword & Shield FAQ (2020-01-24), TPCi Rules Team (2020-02-06)