Certified GKH Gardening Expert
This could be a fertilization issue if you do not fertilize regularly. Also, have they outgrown the container? Are they rootbound? Has the soil ever been replaced or added to? Also check the petals for spots, which can indicate a fungal disease, petal blight.
This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/azalea/common-azalea-problems.htm