My petunia leaves have turned yellow and some of the flower buds have turned brown. The under growth looks healthy. Flowers continue to bloom, but not as much as they use to. I have 2 white petunias and one purple petunia in the pot and 2 white petunias and one pink petunia in the other pot. It only seems to be effecting the white petunias, not the colored petunias. I feed all of my petunias every other day with miracle grow and water. I finally cut the yellow leaves off because I was afraid they had something wrong with them.
You are watering regularly, but do you check the soil before doing so?
The plants should be kept moist but not soggy.
Weekly fertilizer would be a good schedule.
You will have smaller blossoms and less flowers in a more shady location and more flowers in bright sun.
Dead head the old blossoms to encourage more flowering.
Here are a couple of great articles.