If you're not losing weight, there could be several causes beyond the obvious. There are many different root causes, but here are a few I could readily think of:
1. Your nutritional profile is not conducive to losing weight, e.g. there should be a balance of carbs, proteins, fats and fiber.
2.You're body is in a metabolic doom loop that prevents the loss of stored fat. Specifically, our body may be glycemic-sensitive and/or insulin-resistant, which causes extra production of insulin, converting food to fat and locking it in such that the body will not burn it off.
3. Your body is acidic, which may cause the development of additional fat cells, which the body will resist to get rid of until the body is more alkaline.
4. Your body may not be digesting your food properly. Also, if you're eating a healthy Diet as you mentioned, you should be having regular bowl movements. Otherwise, your intestinal tract may still be clogged.
5. You may not be exercising enough or you may be exercising incorrectly.
6. You may have some subconscious behaviors or habits (e.g. eating while watching TV, snacking while talking on the phone, cleaning your plate even though you're full, eating foods with 'hidden' calories, etc.)that you're not aware of. Or, you may be using food to satisfy some emotional need.
Whether the problem is physical, chemical, emotional, etc. the key is to find out what the ROOT CAUSE is so that you can correct it. Once you correct the root cause, losing weight becomes a lot easier.
Note: Refer to my (DeWayne's) comments under "Diet and excessive sweating" for some actions to take and get on the right track.
If you have questions or need more detail send an e-mail.