Troubleshooting Azalea problem
We viewed your photo and cannot say for sure what you are dealing with. It does not look like an insect or disease issue. It is possible you may be dealing with a chlorosis problem or a root issue.
You did not mention where the shrubs are located, how old they are, etc. You can also send more photos. If the azaleas are located next to a brick wall with mortar and/or a cement walkway lime may leach into the soil. Your soil pH may have changed over time. If the pH is too high, then the plant may not be able to uptake nutrients. Azaleas grow best in a pH of 4.5-6 in a moist, well drained, acidic soil in morning sun and afternoon shade. They have a shallow root system.
At this point, test your soil. Results will give you pH and nutrient deficiencies.
If the pH is in the correct range, then you may be dealing with a root problem. Check the drainage in the area and make sure the soil drains well. Mulch should be no thicker than several inches and away from the base of the shrub.