Keep applying penetrating oil over a few days. Should release then. Had it with a bleed nipple on a caliper. The bolt was almost rouding i was having to try so hard to undo it. I kept spraying it with penetrating oil all week and then it undid. Dont use WD40, use proper penetrating oil. I always use a bit of anti seize compound on the new plugs too. Saves them getting stuck in the future.