911 sc plug leads wont come off

[3.0schotrod]

Ok, slightly embarrassing one maybe, just got a 3.0sc, first Porsche, ive worked on many cars and changed hundreds of spark plugs, but I cant get my plug leads off the plugs. Ive tugged on 3 with more force than I would like, but the plug leads aren't coming off (need to change the plugs). What am I missing. Don't want to break anything and cant find any posts suggesting someone else has had the same problem. Maybe I need to eat more spinach, but im really tugging on these things.