Are you sure they haven't dropped their flowers because they're about to grow fruit?
Otherwise lack of insects or wind to spread pollen & fertilise the flowers is the probable answer

Could your greenhouse be too hot? It's currently 95°F today in New York. Any hotter, my tomatoes will abort and they are outside. Ideal fruiting temp for tomatoes is 75°F-95°F. If it's too hot, open some window & get some ventilation in there.

My greenhouse has shading net over the top and still reaches 45 degrees. Chilli plants and melon inside but thinking of putting them out. My tomatoes have been out since May and have enjoyed the weather.

Could be as Bathgate says, too hot. Some of mine have dropped flowers, in my conservatory, despite door & windows open it has been way up in the 80s at times

I did see some where 77F is the perfect temperature for toms, presumably derived from commercial growers where every degree above ambient costs heating money

Thankyou.