Water melons love hot weather, the hotter it gets the better they like it. It was 102 here in TN 3 weeks ago, it has been 95 to 97 for the past 3 weeks, my melons are doing good. No rain here for about 2 weeks soil is dry and dust. Best melons I ever grew it was 115 degrees in AZ. Melons do not like it wet. If your soil is wet that might be the problem. I keep my melons on cement blocks, brick, patio blocks, flat rocks, in TN. Melons grow deep roots maybe your soil is to hard for your melons to grow deep roots. If the end of your melons are turning black they have BER add lime or wood ash. Do you have photos? What is your geographical location? If you live out west you must water your melons.