Are these indoor or outdoor plants? In containers or in the ground? What watering and fertilizing practices have you been following with your azaleas?
An insect could very well be the culprit and I would recommend that you examine your plant thoroughly for any signs of them. The azalea lace bug causes washed out looking leaves with black dots (excrement of the pest) on underside of leaves. For more information on how to identify these pests and for a prescribed course of treatment, please visit the following link:
Another insect pest that might be suspect is the whitefly and more information about this bug can be found here: https://www.gardeningknowhow.com/plant-problems/pests/insects/whitefly-control.htm
Your problem could also be fungal in nature. Azaleas can become afflicted with leaf spot which can be treated with a fungicide to help clear it up faster.
For more information on azalea diseases and pests, please visit the following link:
For a refresher on the care of azaleas, please visit the following link: