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If the summer heat has hit you may need to increase watering.
Have you inspected the plants for insects or disease?

You may want to treat with Neem Oil as if works as a insecticide and fungicide.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Most likely to much Nitrogen in the soil.
You can add Phosphorous which will promote flowering.
Bone Meal is a good way to do that.

Here is a link with more information.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm

Yellow leaves can happen for a number of reasons, but in your case I am wondering when you fertilized last? I think they may be getting too much nitrogen. This can cause explosive growth at the beginning with few to no flowers and eventually causes the plants to yellow and start to die.

If you have not fertilized, then underwatering, lack of nutrients or a fungus are to blame, likely. Increase water a bit, use a balanced fertilizer on the plant and treat it with a fungicide. This should take care of the most common causes of yellowing.

You may also find this article helpful:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

This is likely just the aging of the plants.

Shasta Daisy is very forgiving and you can cut back and prune quite aggressively.

This is not indication of your the health of your soil. A soil test is always a good idea in a new garden or a garden that has been neglected.
This will help you take the guesswork out of fertilizing and soil amendments needed.

https://www.gardeningknowhow.com/ornamental/flowers/shasta-daisy/planting-shasta-daisies.htm
https://www.gardeningknowhow.com/extension-search/

This normally happens because the plant do not get quite enough sunlight and they have weak stems. If you can get them more light, that would be helpful.

You may also have an animal bedding down in the flowers at night. This will create a circular pattern of flattened plants and kill the plants in the center.

You likely have to much Nitrogen in the soil.
This causes lush green plants and little to no flowers.

Adding Phosphorous is a remedy and Bone Meal is great way to do this.

Here are a few links for you.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/too-much-nitrogen-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm