As Schmoodle said- everyone tends to react differently to diet and exercise- I don't know what you're doing to try and lose weight, but keep in mind that it needs to be tailored for your body. It could be anything from you not eating enough, exercising too much for your body type, not exercising enough for your body type
Without knowing what you do- it's hard to pin point the problem
Just keep your chin up and let us know what you're using
Rach