I would guess the issue is the soil. I have made the change to avoid organic matter in my soils
...
My citrus are in basically rocky soil with decomposed granite, pummice and other stuffs like sand. I might have put some peat moss in.
I want to point out that the type of soil that should be used for potted citrus probably differs very much depending on what part of the country you are in.
On the U.S. West Coast, you want rich soil that will retain moisture well. On the East Coast, you want soil that can drain well and not result in too much moisture build up or waterlogging.
In climates with hotter summers, it's possible the plants are getting too much full sun. This can create water stress on them, and the roots may not be as capable of providing the larger volumes of water due to the more limited root space in containers. This should
not be taken to imply that the solution is just to give the plants more water, since that can cause problems.