How long have you been on the healthy diet? If you only recently moved to a better diet, it may take awhile to see results.
Also, sometimes if we eat the same amount, but substitute high calorie "healthy" food (something like granola comes to mind!), we can gain weight.
The best diet that has been studied around the world for many years is often referred to as the Mediterranean Diet. It consists of lots of vegetables, fish, some whole grains and healthy fats such as olive oil. What it doesn't include is processed foods (cereals, crackers, chips etc.) While you can eat some meat on this diet, it is in small amounts.