Welcome, Jenny!
Usually, that's lack of pollination. If bees are in short supply, try taking pollen from the male flowers--the ones without a baby squash at the back--with a brush or Q-tip, and applying it to the female flowers, preferably on a different plant. The best time to do this is in the morning. Good luck!

Many thanks 'Tugbrethil' for your reply. I asked the question on behalf of a friend who was grateful for the answer and was going to give it a go. Thanks again,
Jenny