Could be a watering issue. It's easy to drown a plant in a container.
Could also be lack of fertilizer. What are you fertilizing with and how are you applying it. Container plants only have a finite amount of soil to draw nutrients from and need more careful fertilization.
The other thing that comes to mind is maybe early blight.
What is your (rough) location? Gardening is a local thing and knowing where you are located lets people give better gardening advice.
Welcome to the forum!!!
Ed
