Why isn't my cat losing weight on low-calorie dry food and wet food?

Getting cats to lose weight can be very challenging. Cats are obligate carnivores and cannot use carbohydrates efficiently, so they tend to be converted straight into fat. Any any dry food needs to have a good amount of carbohydrates in it in order to makes it "dry". So, even using low calorie biscuits, it can be very difficult to get a cat to lose weight. (And the bags generally recommend more calories than a normal cat needs.) My best recommendation for weight loss in cats is to change to a completely canned food diet. This website has some good information about cat nutrition.
http://catinfo.org/
Some of the packets are actually also high in carbohydrates, so you have to read the labels carefully. It will seem odd that you are actually feeding a higher fat and higher caloric food in a canned food, but cats tend to lose weight that way. I generally recommend an average of 5.5 ounces of canned food per day for an average ideal body weight of 10 pounds for a cat.
Good luck! I hope that was helpful. Thanks for using PetCoach.