Any question answered...

Well a new chain might help. If the bushings are damaged (als it rot of looks like it if you look at the pin that is almost out of the tooth there) it can be stuck. And don't use WD40 but use a good chain lube. In this case, get a new chain and see if it fixes the problem. It's only a few pounds and you can probably get it fixed with that.

It's the photo, all pins are in fine and I don't know how the bushings would have got damaged as it's a new chain that worked on my old cranks.
WD40 was a last resort, and it hasn't worked. It was cleaned an re-lubed afterwards.