When we cook pork roasts on our Weber Family Q, I follow the instructions precisely, but sometimes the skin does not always fully crackle.
We have found that placing any skin that has not crackled properly in our air fryer solves the problem in minutes.
Likewise, when we have the leftover pork and roast veggies for dinner the following nights, we place the roast potatoes and the pork skin, whether fully crackled or otherwise, in the air fryer to reheat.
The crackling is as crisp and as crunchy as those packets of porky crackling bits the supermarkets sell.
Other people have said that they remove the whole skin and heat it on high in a microwave or a wok, but the air fryer is very quick and easy to use, and there is not oil and fat everywhere to clean up.