I have several of each of crape myrtles (lagerstroemia) & trumpet creepers (bignolia) and although they have been in my garden for 2 or more years, most of them have never produced flowers. I live in central France and enjoy a sunny warm climate in summer. The winters can be cold so I do protect the plants at that time. My garden soil is very rich and middling Ph. I seem to have half the world's population of bees & butterflies around my abundance of other flowers.
In detail, I have 6 trumpet creepers of various sizes and ages but only one regularly produces flowers. the oldest and largest is 5 years old, wrapped around a tree and is 5 metres tall - last year it had 2 or 3 flowers, this year none. The rest never.
I have 8 Crape myrtles but 4 of them are still small and this was their 1st full year in my garden so I'm not concerned there.
The others are 3 - 4 years old and middling sized. One has just come into flower for the 1st time this year. The others - nothing.
Can someone give me a reason for this?