The plant is probably not getting adequate sunlight. Petunias, verbena, calibrachoa (million bells), marigolds, geraniums, zinnias all need AT LEAST 6 hours of DIRECT sunlight a day.
If they are getting at least 6 hours but there is still a noticeable decline in blooms, it may be due to the fact that when you purchased the plant it was getting 12-14 hours of light and it is adjusting to the lower light conditions. Petunias, particularly, want as much sunlight as possible.
Consider moving the plant slightly so as to expose it to two or more hours of additional sunlight. After moving the plant to a sunny spot prune off the leggy, stringy portions. This will encourage bushier growth in the plant.
Impatiens, coleus, and begonias need less light. These plants are shade plants, but they do need a fair amount of reflected light to keep blooming. While they do not want the hot, late-afternoon sun, they can tolerate some morning sun and would prefer to be in a bright shady spot all day.