Mk7 , spark , but no go!

No . 1 cylinder has a great spark , but is not functioning . Tried three good plugs , no go ! Removed the lead while the engine was running , sounded no different . Is this a valve problem ?

I would try a compression test first.

Compression was 110 before running it . Now down to just over 100 . That cylinder is not contributing anything .

And how is the rest?
100/110 is a bit low, but high enough to do something.
Stationary disconnecting one spark does not give a lot of difference on a 6 Cyl anyway.
What happens when you disconnect another cyl ?

The rest are around 110 , it’s a 7 -1 engine , so ok . I,ll give that a go tomorrow