Be sure that you are not overwatering the tree. They need, at most, an inch per week during hot weather, but it is possible to overdo it and drown them. Yellow leaves can appear in both drought and flood. It sounds like the tree was stressed last year, and that it is still suffering. When you water too frequently, even for tree that likes moist soil, it will not make as great an effort to put out new roots and become established. When it doesn't become well established, it suffers more in hot weather because it doesn't have the root system to support itself. If the winter was tough as well, you may lose the tree just because it has struggled for over a year. Hold off on the watering until the top two inches of soil is dry. If the tree still struggles next summer, it may need to be replaced.