Why is my Penstemon dying?

Jonathan-10

 I have two Penstemon planted in the same bed. One is thriving the other seems to be dying. They were planted a year ago, and I first noticed the one with the problem a few months ago.

I see no sign of insects, no fungus, I’ve dug around the roots and founds no Vine Weevil grubs or pest of any kind. I’m at a complete loss! Would greatly appreciate any advise. See photos above.

Fire

Could anything be peeing on one of the beds? I believe it is quite straight forward to take cuttings.

Fairygirl

Is there something else nearby - a large shrub or tree perhaps - which is sucking up all the moisture round it? Even a wall, which is preventing some necessary moisture getting to the plant properly?
In long dry periods like last year, that can be enough to make the difference.