On Mon, 15 Sep 2014 11:46:53 +0100, Indigo
wrote:

Younger penstemon plants also tend to flower more prolifically than
older ones that have got a bit woody at the base. You could try growing
a few new ones from cuttings and see how they fare in a different site;
they're quite easy in a gritty compost mix, I find.

This I think is the reason why my penstamons have not flowered this
year. They are last years growtgh which did not die back last winter
so I left them. Thanks Sue. That certainly seems to be the most likely
answer for me.