In addition to the good suggestions about water below, you might try fertilizing with compost and composted manure. Since it is too late to amend your soil now that it is planted, you might consider side-dressing your existing plants. After you take them out in the fall and before you plant next spring, work these into your soil. Additionally, www.solutionsfromscience.com has a great product called Protogrow. It is fermented kelp and fish and is much better for your soil, plants, and you than MiracleGrow. This is what I have been doing and my plants are huge and producing like there is no tomorrow. I had to do a squash run yesterday to deliver extra squash to people who wanted some and were having similar problems as you are.