It happens when they haven't been pollinated. The easiest way around that is to take a male flower with plenty of pollen, tear off the petals and push the sticky-up bit that's left into the hole bit of the female flower and wriggle it around. I'm sure you can use your imagination here
When there is plenty of pollen you can tap it in.
One male flower will do a couple of female flowers. Both need to be at the right stage, so the male with plenty of visible pollen, the female fresh and unfaded but fully open.