Likely lack of pollination. You get to be the bee.

There is a FAQ here that explains it all plus another on how to do hand pollination.

Dave

Here is a link that might be useful: Vegetable Gardening FAQs

It's easy to pollinate your own plants. Look up what the male and female flowers look like and you can do it in seconds.

Thanks everyone.....I shall go out there and
Buzz-Buzz", we have not had very many bees the last few years.

I'm getting the same thing on some of mine, but it's when they are already4-5" long. The herb garden is near-by, so no lack of bees.
I'm wondering if the leaves are too thick for the bugs to get in and pollinate. They are thicker than I've ever seen them! Should I prune a few back? NT

Mine just simply don't have any female flowers, and the plants are growing straight up.

but it's when they are already4-5" long

That is BER - blossom end rot - when they get that big. It's fairly common in squash just like tomatoes. There is a FAQ available on BER.

Female flowers usually follow the first males by a week to 10 days.

Dave

Glad this is happening in my garden because I know that in a month they will be stacked up on my front step as big as my arm.