I planted some Hubbard squash around Aug 1.  They have been growing vigorously and are now about 15' long.  I get dozens of male flowers and a few females.  Each plant has a nice large well formed fruit but the other female flowers never get more than about 1/2" long then turn yellow and fall off.

Should they get more than 1 fruit per vine?

You might have a stink bug problem ! My summer squash did that and I found out that the stink bugs liked eating the flower portion of the squash and it would fall off and die .  This might not be the problem but you could find out by taking a spray bottle with a few Tbl spoons of dish soap and water in it and spray them down , the bugs don't like the soap and will leave it alone .  If you have egg clusters on your leaves , I'd bet this is your problem !

Oh how I hate "squashbugs".  We got a terrific crop of zucchini once I got rid of the bugs.

Try hand pollenating the female flowers, use a q-tip.

They're not getting big enough.  They are turning yellow while still tiny.  Could extreme heat and drought cause it?  I don't have much time, only 3 weeks or so until frost (even though it was 90 yesterday )