Why with diet and exersicse can I not lose weight?

Too many calories. Minimal weight loss despite exercise and eating a healthy diet is most likely due to consuming beyond the calorie limit required for your body to burn fat. You have to eat about 3500 calories less than what you burn over 7 days to lose just 1 pound! Track your calories by weighing and measuring your food and using the label information. Calories from beverages add up quickly too. .
Calories in and out. Weight loss is a function calories in and calories out. As long as your calories out (exercise and metabolism) exceed your calories in (diet) there will be weight loss. However, see your doctor to be sure there are no conditions in your health that would preclude you from exercise and diet, or other medical conditions that could also result in weight issues.