It is often referred to as fruit abortion and is usually due to stressed growing conditions ie: excess nitrogen, excess prolonged dry hot periods, insufficient watering, etc. Often a really deep watering will eliminate the problem. Usually as the plant matures and if the conditions are improved the plant will adjust and produce.

Dave

PS - just noticed you said " Bevause the vine and leaves continue to grow vigorously, iiI'm thinking its it's too much and not enough P and K". P and K levels are seldom a problem but too much N may well be the problem. Adding additional fertilizers couold only make the issue worse.

Are these in containers? Is the sun shining on the container? Squash roots die when the soil gets too hot, and flowers are the first to croak. Also, the plants stop absorbing water when the temperature is in the 90s, even when the soil is moist. So the plants will be stressed in any case.

What's a hybrid volunteer? A volunteer from a hybrid may not be true to that hybrid. I suspect you don't even really know what you're growing!

My comment above seems not to have registered as a "comment". The thread still reads "Be the first to comment". Maybe if someone sees this, they could flag it (I can't flag my own post), so a moderator can be apprised and try to fix it. I've seen this happen a number of times, actually.

The environmental stress of high heat can abort pollination. Also, in hot weather, plants might be running short of water.

If you used an all-pupose fert at or shortly after planting, you don't need more.

I have no idea, Maybe because it is a volunteer and probably cross pollinated the plant is confused. I usually get some thing from volunteers tho.

Update.

I havent gotten the fertilizer yet but I noticed something else happening on the plant.

The one fruit that i mentioned was pollinated, grew a bit then got stunted is now starting to rot.

I'm thinking this might be blossom end rot but wouldnt that start from the ends first?

You should understand that your post was locked down by Houzz, and for many days, responses weren't being recorded. That has now been fixed. See https://www.houzz.com/discussions/when-a-comment-isnt-a-comment-dsvw-vd~5384058

BER can develop at any point on a fruit. The blossom end is most common but your photo is definitely BER.

Dave

PS: I 'think' that tagged as a FAQ means that there is a rather lengthy FAQ here (somewhere) about this question. And it is also a very common question posted. Problem is we can't seem to find the FAQs since the reorganization and even using the recalled titles of many of them don't pull them up with the Search as it used to. The search does pull up 50+ previous discussions about this but no FAQ.

The early apparent growth is normal without pollination.

But then, the stop and abort is lack of pollination.

So get busy and be a bee. Transfer pollen from the male to the central peg in the female flower early every morning.

Oh wow, thank you for the responses everyone. I didnt mean to ignore any concerns that you giving me. I am just seeing them all now thanks to the lockdown. I will get back to the other posts and give an update tomorrow when I have a moment

Two years late, but did you manage to find out what the problem was? None of the answers above have actually answered your question - "the fruit will turn yellow and shrivel up before it even gets big enough to open its flower for pollination". Mine are the same. New growth, just tiny baby fruit, maybe a week away from being pollinated and they actually start off as yellow. Would love to know what I'm doing wrong. Not too hot, being watered carefully.

I am having the same problem. I have so many babies and they all turn yellow and folk off before the flowers can open for pollination :(

iirc blossoms are only open for business in the morning. Do you see bees in your garden? If not it may be up to you. IF you are using insect sprays that could be part of your problem?

My butternut did not produce any fruit for a solid month (it finally is now) - it was really hot. I did not see when the fruit died but the hot comment got me thinking.....