Hi all my first posting here

I've been planting watermelons, zucchini, pumpkins, and acorn squash for several years. This problem has happened to some extent every year, and it's happening again...The plants are growing fine, but when they begin putting on fruit, the baby fruits turn yellow and fall fron the vine. on the zucchini the beginning of the fruit stays green and keeps growing, but the tail end turns yellow and soft, eventually going rotten. On everything else, the fruits just stop growing first...a few days later, they are soft and either turning yellow or they just fall off. Especially the first few fruits that show up on the watermelon, they all do this. Later on in the season, I usually end up getting a few watermelons to grow. By then they are so far behind that I don't harvest any til mid-september! The zucchini I have gotten about half of the fruits so far this year, the other half get this problem and get tossed. Can this be fixed?