Certified GKH Gardening Expert
It sounds like they are a bit too moist. They need to dry out between waterings. In fact, they prefer dryer soils once they are mature.
Be sure to let them dry out a little between watering, and treat with a fungicide.
This article will offer information on fungicide use: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you with Zinnia care: https://www.gardeningknowhow.com/ornamental/flowers/zinnia/growing-zinnia-flowers.htm
It looks like you are using topsoil in your container. This doesn't work well for many plants; consequently, potting mix (no soil included) leads to much better growth.