You may have to hand pollenate using a small paint brush or Q tip...

Do you have all male flowers. They look as if they are long stems with a big yellow flower attached to them.

If this is the case you need to let the sun and warmth in and this should then produce fruits.

I had this problem earlier in the season, but mine have now cropped really well. By the way mine are grown in pots, so I was able to move them to a sunnier position.

you don't say when or where you planted your courgettes, are they inside or out, were they planted early or later.

Also as Moongrower says "You may need to hand pollinate"