It may happen for many reasons. Couple of them are below-

1. Adverse weather conditions: Hot and dry weather may abort the pollens results in poor or no pollination. High humidity make pollen more sticky and won't transfer well. Also cool and cloudy weather reduces honeybee activity.

2. Lack of Pollination: Watermelon mainly depends on honeybees for pollination. In general if the pollination does not occur, female flower will grow initially but fall off later. So if your garden is lack of bees you should go for hand pollination.
Hand pollination can be done by using soft brush or cotton swab. Run the soft brush on opened male flower and then on to female flower. Or gently rub the yellow hair-like structures of the male flower against the middle parts of a female flower. Repeat the procedure for 2 to 3 days. (Both flowers look similar but male flower grows on long peduncle and female flower on short peduncle but it has an ovary in the shape of the fruit).