You have to balance both diet and exercise. If you increase excercise what you could be seeing with no weight loss is muscle gain and fat loss but because muscle weighs more than fat it may not show up on the scales.
muscle does not weigh more than fat, muscle just takes up less space than fat. 1kg of muscle is the same weight as 1kg of fat.

clover, maybe you aren't feeding your body enough? I know if I drop below a certain amount of food in a day my body doesn't like it and nothing shifts.