it sounds like they are not being pollinated when the flowers are open use a paint brush to get the pollen from one flower to another

when this happend to me i had to self pollinate,to do this you first have to sort out the male from the females,this is easily done,the females have a small fruit behind the flowers where as the males dont.Find a male flower and cut it off about an inch behind the flower just leaving a bit to hold,care fully pull the petals off so you can see the male stem inside,push this care fully into the female flower making sure the stem enters the female part of her stem,i hope this helps its very easy to do

If you grew the plant from the seeds of a butternut squash you bought in the shop there's a good chance it was a hybrid, in which case it won't fruit. Growers of fruit and vegies do this to make sure you buy their stuff instead of you growing your own.