Laura - I'm a gardening novice with the same experience with buttercup squash in my backyard garden. I think the issue is one of the following - (1) the female flower did not get pollinated or (2) potentially it was cross pollinated (I happen to have pumpkin nearby and am wondering if this is a natural result of cross pollination). There are no other obvious indicators, so I think it is one of these issues, most likely #1. I am a bit surprised by how "encouraging" the fruit grows and then just stops, yellows and falls off - even with mature fruit next to it. Despite the presence of numerous bees and bugs, it has happened to more than 50% of the fruits on my vines.

Have you heard or found anything different? I may resort to hand pollinating some if not too late in the season.

Mike Z