Add the frozen meatballs straight into the slow cooker. Do not thaw them first.
In a bowl, combine the barbecue sauce, grape jelly, Worcestershire sauce, garlic powder, onion powder, black pepper, and red pepper flakes.
Whisk the sauce until it looks smooth and glossy.
Pour the sauce over the meatballs in the slow cooker.
Gently stir so each meatball gets coated with sauce.
Cover and cook on low for 4 hours, or on high for 2 hours.
Stir once around the halfway mark.
During the last 30 minutes, remove the lid completely to let the sauce thicken and cling.
Cook until the meatballs are hot through and the sauce coats the back of a spoon.
Serve straight from the pot to keep everything warm.