Too much water I would imagine just because of all the recent downpours we have had. Plus if you have birdhouse gourds (and most others) not every gourd will make it anyway. I doubt it is a serious problem, I am sure you will still get more than a few. It is still pretty early in the season they might not even be getting pollinated. If your gourds have yellow flowers they are pollinated by bees in the day and if your gourds have white flowers they are pollinated by moths at night. Try pollinating a few by hand it isn't too hard. Just when a flower is open take off a male flower and remove the petals so that only the stamens in the center of the flower remain and rub it all over the pistil of the female flower. Good Luck.