The flowers are not getting pollinated. Insects or Bees do this. Are they outside ? Should be plenty of Hoverflies
to take an interest in them.
Otherwise, use a piece of dry cotton wool and go round the flowers gently rubbing each flower then going on to the next one. Thats hand pollination.
Used to be done with a Rabbit's foot when people caught Rabbits for the stewpot.

Thank you I will try that ,but the flowers look like they are drying out.

Then spray overhead in hot weather and damp down the floor.
But NOT when the greenhouse is in full sun or late in the evening, see if that helps, its what I do and the bush ones are already giving us ripe fruit with lots of toms on the cordons, much better than last year

As answered elsewhere, tomatoes do not need insects for pollination. Nor do they need cotton wool. They do it naturally .
Environmental problems cause the flowers to drop off.

Watering from above using a can with a fine rose as soon as the first flowers appear has always worked for me, its said that shaking the plants can also help ...

What a selection ! I will carry on misting mine in the evenings, as they are in the greenhouse where there arent any insects, give them plenty of water in the deep pot saucers, not in the soil as it makes the stems go rotten, and are producing fruits.

Thank you for all your comments, my plants are outdoors and I have done as suggested now I must sit and wait and hope, this is the first time this has ever happened to me

Maybe because it is hotter this year and the air is dryer