Tonnes of flowers and none?

I have 3 Sorbus Sargentiana that I planted about 15 years ago. They were the biggest bare rooted I could find to buy so are now quite substantial.
While answering another question about a tree ( think it was the same species and variety) on GC,I took a closer look at my trees.
Really odd, they are all within about 30 ft of each other, ie all growing in the same soil and under the same conditions.
Normally all of them flower very well and consequently groan with berries as below.

However!, upon inspection one is as per usual and has loads of flowers the other 2 none! I know late frosts ( did not have a severe late frost as we can do some years) are what can cause problems but they are basically next door to each other. The 2 with no berries look perfectly healthy but any ideas why 1 is covered in flowers and the other 2 zilch?