Answer: You didn't mention whether there were any trees nearby. Tree roots can absorb incredible amounts of water and if there are any around, perhaps that's why the soil is dry. Small plants can't really compete with thirsty tree roots.
If trees are not a problem, perhaps water is evaporating at a rapid rate during hot weather. You can slow evaporation by covering bare soil around the plants with 2"-3" of organic compost.
Finally, it's possible you're not watering deeply enough to keep the roots of your plants happy. Try applying water slowly so that it soaks in and thoroughly wets the root masses of your plants. (Frequent, shallow watering will keep the roots in the top few inches of soil, which can dry out quickly and cause plants to wilt.) To check the efficiency of your watering practices, dig down into the earth after watering to see how deeply water has penetrated the soil. You may be surprised to discover you're not applying enough water at any one time to percolate down to wet the root systems of your plants.
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