Before,I had planted phlox and shirley poppy seeds in the same area of dirt but they didn't sprout.
The seeds were planted in clay soil,could they have been the wrong type of soil?
The area gets full sun in the morning,and the seeds were probably planted in mid-late summer outside.
Mid to late summer is more than likely the problem, but do know that Phlox like nutrient rich soil. I would suggest working some compost into your planting area before you sow your seeds. Hope this helps.
Not enough info provided, as noted by ecrane3.
Were the seeds planted or scattered? (If scattered, they may be prone to blowing away or washing away, or being carried off by birds or insects.) If planted, how deep? Were the seeds kept moist? Can be more difficult to keep planted seeds moist in outdoor plantings. On the other hand, if torrential rains caused water to stand in this area for a long time, the seeds may have rotted.
Soil quality relative to what that plant prefers would affect later growth. In this case, it is said that the seeds didn't germinate so it sounds as though there is some other cause.
Less likely causes may be planting immature or infertile seed, or even thinking you have received seed when it is nothing but chaff... probably not likely, but it happens.