Certified GKH Gardening Expert
It could be overfertilized. The only way to know will be to test your soil, which should be done before fertilizing to make sure that it needs it. It could be overwatering, or disease, too. A soil test will confirm whether the nutrients and pH are in range. If they are, then it could be disease.
Here are some articles that will help:
This article will help you with the care of the flowers: