The most common cause is lack of pollination. I would grab a cotton swab or small paintbrush and dip it into a male flower--one without a small courgette behind the petals--and transfer some pollen to the female flowers.

Thanks; just wondered how the courgette forms if there has been no pollination?

Well, when each female flower forms, there is already a tiny courgette attached, but it doesn't grow much without being pollinated. It's the plant's way of being ready when the bee arrives, but not wasting too much energy on something without seeds. Other flowering plants do the same thing, but their unpollinated fruit die off much smaller.

Thanks ever so much will get my paintbrush out.