Female pumpkin flowers wilting before opening
 

Does anyone know of possible causes for why the female flowers on my
pumpkin vines keep wilting and falling off before they even get a
chance to grow enough to open? I have one pumpkin doing very nicely,
growing a lot. But not one of my other female flowers will blossom.
I have no problems with males; they are abundant and doing just fine.
I have 6-7 vines, 3 of which are very long and seem to grow at least a
couple inches every couple days, and I'm in Wisconsin. I don't seem
to have any bugs, and the leaves seem to be fine, not yellowing
prematurely or anything like that.

Hopefully that's enough information to go off of. Thanks for any help
you can give.