Garden soils can make such a difference in how quickly flowers will cover the bed. With all the rain we had earlier, poor drainage is often at the "root" of things with water logged soils and limited soil oxygen present. Good root development counts on a balance of both. Overly wet soils will prevent roots from growing deeply so when our weather moderated, there were no roots down deep to support the flowers with the water they needed. This was not as noticeable on older established flower beds that have a good amount of organic matter, yet it still occurred with our excessive spring rainfall. New and newer beds did not have the benefit of that organic matter that will help keep the soil open and allow soil oxygen into the soil profile. In these beds gardeners are seeing the flowers wilt daily with strong sunlight and then recover overnight. One more influence on root development is how well the root ball was disturbed coming out of the transplant flat. Planted as it came out the flat "as is" limits the outward development of the roots.
Since we did have all that rain earlier another challenge is making sure there is adequate fertilizer left for flowers to develop into mature plants. Cold wet soils early often limit plant uptake from the garden soil. Once the soils warm and dry out, it is easier for the plants root systems to function correctly. Older flower beds fared better again since all the organic matter is Mother Natures' slow release fertilizer, being available whenever the flowers need it. If the flowers have "stalled out" consider side-dressing with general purpose flower or tree and shrub fertilizer. Applications can be made right on the surface of the soil and watered in to get flowers going again. If you are going to work the fertilizers into the soil, do so shallowly, you don't want to damage any roots. The sooner the fertilizer gets activated with natural soil moisture or the watering provided, the sooner it becomes available to the flowers. Liquid foliar fertilizers are yet another choice. It will take a few days before you see any improvement so be patient.
Weeds can also compete very well for nutrition, growing right along with your flowers. It was very hard to keep up with all the rain earlier and the whole time those weeds were there; they are using food meant for the flowers. That can easily set the flower plants' development back too.Depending on your weeding practices, there is likely another round of weed seed germination going on as well, so take care of them while they are small.