When and how did you prune/dead-head?

I pruned all of them back into the wood just above the point where bulk of the new growth was showing in late March. I am pretty sure that I fed most of them with a slow release fertiliser but some I may not have done. Generally, I do dead head quite fastidiously.

Too much leaf and not enough flowers would normally suggest too much nitrogen relative to potassium in the soil. Might this be the case? Perennials don't generally need much feeding, so it is possible that either (a) you have overfed them, or (b) you have underfed them, or indeed (c) you have done everything you could possibly do and the Penstemons just weren't happy last year. Do you mulch your soil with plenty of organic matter? Oh, and what about their position? If they are not getting enough sun, then they would certainly produce lots of vegetative growth at the expense of flowers.

Hi. I grow quite a lot of Penstemons & never feed them, apart from the ones I grow in tubs but these only get fed by way of repotting with fresh compost, some of them will grow in semi shade but usually flower better in the sun.