Get rid of the plants or the soil? Sounds as though you might have an imbalance with more nitrogen than potassium. Try giving them a dressing of sulphate of potash and lay off the nitrogen for a while.
You can cut the plants back quite hard if you haven't already done so - leave some buds near the base though.
Then if they appear to be responding a bit you could water them every few weeks with tomato fertiliser later on in the season.

Hi, I agree with Steragram, Derek.

Also check the sun exposure. Most kinds of penstemon need at least 6 hours of direct sun a day--at least on the days that the sun shines :)

I agree, I bet not in a sunny spot and its too shaded.

How long as the plant been in for, as if only new it would make a lot of new growth