My Romanescos did that this year too, and we had water aplenty. I thought mine had pollination problems, because they eventually turned yellow if I left them on the vine. The ones I picked that looked like yours tasted fine though, but did not keep as long as my dark green zuccs.

Thanks for your reply. They don't seem to properly mature, the flowers don't drop off, the ends yellow. Several little ones never made it past 3-4 inches. I'm really disappointed. I started them from seeds in the packet. Keep hoping the continued crop somehow starts to develop properly. Summer isn't over yet! I never thought zucchini could fail!

It must be the variety maybe? Had the exact problem you describe. My other two types of zucc and all my other squash were unstoppable.