I'm assuming the little tiny watermelons are the females and they are not getting pollinated properly. I usually pollinate by hand using a small soft bristled paint brush to transfer the pollen from the males, which are the flowers without the tiny watermelons, to the females. It seems to work well.

My melons grow but develope what seems to be blossum end rot like tomatoes do.
Dan

Is there away to avoid blossom end rot? I am new to gardening. Thanks.

More Calcium? Or proper Ph and nutrient uptake.