Flowering dogwood (Cornus florida) is hardy in Zones 5-9. If you live in the colder end of the range, your trees may lack flower-bud hardiness. Northern nurseries often sell stock that is southern-grown. With some plants, it makes no difference. In the case of flowering dogwoods, it does. Make certain your dogwoods came from a local source. If that's not the problem, several disease and insect problems may be at work. From borers to anthrac-nose, stressed dogwoods are susceptible to many foes and woes. Make certain that your trees are planted in the right location and getting proper care. You might consider replacing your flowering dogwood with pagoda dogwood (C. alternifolia), hardy to Zone 3, or kousa dogwood (C. kousa), hardy to Zone 5. They are more disease-resistant and hardier than flowering dogwood.