I kind of researched this and was given 2 possibilities.
One was blossom end rot. I have 50lbs of calcium carbonate (oyster shell), so I sprinkled that around the roots. We'll see in a few days.(I also heard you can stick a few TUMS down in the roots?!?!)
The other was lack of germination, or was that fertilization??? I think the latter. Then you go out with your little paint brush and tickle the male flower, then the female flower (you've heard about the birds and the bees, right?) They say it's a lack of bees. I'm going to plant lavender in my garden for next year! They bring PLENTY of bees!
My yellow squash are doing GREAT!!! The Zuks are the ones not producing! Good Luck! Nancy

try adding lime (i used e.b. stone organic). i had both blossom end rot for sure, and i suspect pollination issues as well - my male and female flowers don't come up at the same time.

anyway, what little zucs i got sure are pretty and the yellowing cleared up within 2 days - very noticeable - nice big pretty flowers!

hope this helps and good luck.