I have a cat with particularly smelly poo, is there anything I can do to help?

The most recent addition to our home is an adorable small grey tabby cat (AKA Sammy). She has the same diet as out other two cats, but her poo is a horse of a different color, it is very disgusting. She has been wormed with several different treatments, so I doubt that is the problem. We never had a problem with our litter smelling so bad until Sammy moved in. We have 2 other cats who have bad, but normal smelling poo... are there any ideas why Sammy is having this issue?