Janet, We grow Heavenly Blue here in Sussex every year. It used to flower abundantly, on a sheltered fence or a wall, facing more or less north-east. But we have had very similar problems with very poor flowering (but abundant growth) in the last three years. Last year and the year before flowering was extremely disappointing. This year the spring here was late, cold and wet but summer was sunny and warm. Flowering was very late indeed, and rather poor. (We have also had problems in the early part of the season with poor leaf development and some whitened leaves - almost certainly cold, we checked that it was not nutritional.) Our seed comes from Kings.
Ipomoea 'Star of Yalta' (seed from T&M), smaller purple flowers, has in comparison been very reliably floriferous each year. And Ipomoea quamoclit pretty reliable too.