The main issue with watermelon is pollination even though it produces a large number of male and female flowers. Pollination depends on bees, wasps, etc. In general, if the pollination does not occur, the female flower will grow initially but fall off later (small fruit appear to develop but dry off later). So if your garden is lack of bees you should go for hand pollination.

Hand pollination can be done by using the soft brush. Run the soft brush on the opened male flower and then on to the female flower. Repeat the procedure for 2 to 3 days. (Both flowers look similar but the male flower grows on long peduncle and female flower on a short peduncle but it has an ovary in the shape of the fruit).

https://extension.udel.edu/weeklycrop...