I have two questions whose answers may
help solve this situation.
1) Where were the squash plants transplanted *from*? Were they in a sun-protected location? If so, were they hardened off before being put into full sun?
Plants can get sunburn.
2) What were you fighting with the neem oil? What critter(s) had you seen that needed neem? It's considered Major Artillery in the organic armamentum, something to be used when jets of water, soap spray, hand-picking, and other methods have proven to be ineffective or impractical. What was the enemy? Where, how many, how often, etc.?
Be aware that spraying neem is harmful to honeybees and other pollinators, so it's recommended to spray in late afternoon--dusk, if possible--so that the pollinators have "gone home" for the day. That way, the neem can work overnight on the bad guys and evaporate (volatilize, the components that are dangerous to bees) before the next morning.
If you could add your location to your profile, it will also help. The sun is of different strengths at different latitudes, from the Azores to Alberta.
Because you're putting out squash plants in April, I'll assume you're in the Northern Hemisphere, but that still covers a lot of ground. Help?
Sunset Zone 17, USDA Zone 9