Take stem cuttings in late summer or early fall and root them in moist sand. When new growth doubles the size of the cutting, it can be potted up into the same kind of soil used for the mother plant and grown on the windowsill. Use fertilizer half strength until the plant is large enough to repot. The plant could be overwatered. If you are sure this is not the case then your tap water could be the problem.
Citrus plants are very intolerant of the salts that accumulate when they are watered with tap water.
In addition to the yellowing leaves you will often see crusty accumulations of salts on the pot. The plants will decline and eventually die if the problem is not corrected. Pour enough into the pot to run out the bottom, empty the saucer, then repeat twice more. This leaches out the salts. Your plant might be under attack by sucking insects such as spider mites, white fly or scale.