My kumquat tree has pale green leaves with yellow blotches. It grows very slowly and only produces a few fruit every year. It is planted with a lemon tree and a key lime tree. All of the trees are approximately 10' apart. The other 2 citrus trees are doing fine. They all get the same water, sun, fertilizer and the bed is weeded regularly. The soil level on all of the trees is about 2" below the graft. I don't know how to fix the problem.
This could be nutritional in nature or something more serious.
I have listed several links that will help identify the cause.