Welcome to GoY. I can't see anything in Hassayon about not making flowers. I have 1 fruit tree out of ten that has not made blossom this year. It has never performed properly anyway. Perhaps the severe winter has caused lack of flowering on your plum tree. Maybe a bit of fussing with feeding etc might have a good effect and avoid dryness. Water generously if there is a lack of rain.

Thank you - I am still hoping some will appear!( though there is no sign!)
Good job I have a stock of jam left from last year!