On Friday, August 4, 2017 at 1:45:17 PM UTC+1, wrote:
That explains a lot
My Sour Grapes had wonderful flowers the first year but no more. It has fantastic, lush growth but no flowers. I shall take cuttings. It may be an old thread but it has helped me a lot.

I've googled non flowering penstemons because I have exactly the same issue.. Mine are young plants in full sun in well drained soil. Sour grapes is the worst offender with nary a flower but others are similar. Have fed with high potash to try and encourage them but no joy.
Dave
Oct 2018