Impossible to tell with info provided. Likely not pollinated if small. If large, more info.

Dan

Agree with the above. A FAQ and lots of discussions here about the two most common causes and how to prevent them 1) insufficient pollination, and 2) BER.

Dave

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

You might also have squash vine borer.

You might also have squash vine borer.

Not in this case. SVB attack the plant. They do not affect the fruit. And the damage to the plant from SVB is quickly obvious.

Dave

Are they covered in fungus that looks hairy and black (probably not or you would have mentioned it)? Because I lost several to what is called "wet rot" ( fungus Choanephora) which shows that as one of the symptoms.

Here are photos of different types of cucurbit fruit problems (with links to more info):
http://aggie-horticulture.tamu.edu/publications/cucurbitproblemsolver/fruit/

thanks so much.

The only thing that looks remotely like it on that site is blossom-end rot. Even that isn't quite it, I think, because some of them are "squishy" all over, not just the blossom end.

This is only happening to rather young fruits, not older ones. The rest of the plant and some other fruits on the same plant look fine and develop normally.

WAG from paucity of info: not pollinated. WAG.

Dan