The brown leaves could be just the natural aging of the daisies. The daisies also do not like to sit in soggy soil, so check to see if the soil is to wet. The ground should be well draining and dry slightly between watering. If the soil is to dry, you can also see leaves dying off. With the warm summer temperatures you may need to increase watering.
If you can use an all purpose fertilizer and give the plants a light feeding once a week while they are blooming.
Here is a link about care requirements.
https://www.gardeningknowhow.com/ornamental/flowers/shasta-daisy/planting-shasta-daisies.htm