A popular artificial sweetener has become a victim of its own success. Makers of the key ingredients in Spenda say theyre having a hard time keeping up with demand. London-based Tate and Lyle PLC, which makes the ingredient Sucralose, says demand is so high, it will not take on anymore U.S. orders until 2006. Thats when production of the substance at its Alabama plant is expected to double. Demand for Splenda increased after Atkins Nutritionals endorsed it in its low-carb diets.