Hello and welcome to the forum. As Tinyplanets has said, we have a few threads on weight control so you could use the search box to locate them.
Regarding light/diet dog foods they tend to be dry and have low fat (10% and lower) and low(ish) protein. To achieve this, they often contain high levels of carbohydrate in the form of various types of grain. If these are the types you have used it could account for the problems your dog is having. Much depends on the grain used and the percentage of the whole recipe.
There are fewer low fat grain free dog foods but not so many as the grain containing ones. Many of them contain potato as the carb source.
My experience is that weight loss is more easily achieved with low fat (circa 10%) and higher protein (above 26%). Also, it is very much about portion size so you must weigh the food accurately. It can then be gradually reduced in stages of 10% until the dog begins to lose weight.
One new product that springs to mind and might be useful is Cobbydog cold pressed because it is grain free, has low fat (9%) and higher protein (33%). Cold pressed food needs to be weighed accurately because it is heavy. I haven’t checked their recommended daily amount but with other types of cold pressed foods it is usually 1% to 1.2% of the dog’s desired body weight.With a dog that needs to lose weight, start at 1%, divided into three meals. If interested you could contact the company for advice. Goes without saying, no extras and try to up the exercise a little.
It is possible for your dog to lose weight but it could take a while - gradual weight loss is the goal. We can help you to search for something if you return to your thread and tell us what you are looking for.