You could try to 'stress' the plants just a little bit by cutting back on the amount of water you're giving them. Maybe water every second day instead of every day? Don't let them wilt completely, but if the leaves sag a tiny bit in the afternoon sun that won't hurt anything. Plants will hurry up and make seeds (which are inside the squash in this case) if they think they're going to die soon.
Once the female flowers start to appear, gradually increase the water again so that your squash will size up. Once they're in production they should keep producing female flowers.
