If it's only been six weeks, then it might be worth persevering a little longer, and perhaps feeding just a bit less of it. See if they adjust with time. If they don't though, then it's probably wiser to look at a different food.
I'd be a bit cautious about the so-called diet foods though. The thing that puts weight (fat) on dogs the most in dry foods is not actually the fat content but the carbohydrates. Dogs have evolved, and are well adapted to, consuming diets that are in the vast majority made up of meat and fat. Fat is for instand energy, meat protein for maintaining muscle mass. But carbs - well, dogs don't actually have any evolved need for carbs at all. And so, they're usually just stored as fat.
The problem with many diet foods is that they cut out meat content, cut out fat - and give the dog a load of carbs. Which it then prompty stores as fat. Not exactly the aim

There are but a few 'senior' or 'diet' foods on the market that don't do that. The only one I can think of just at the moment is Canidae Platinum.
Another possible option is to try one of the high protein grainless foods instead. Something like Innova EVO (which also has a reduced fat version). What's great about those foods is that they greatly reduct the
carbs in the diet - and although not many people use them with the stated aim of a diet food, many do report that their dogs do lose weight (and gain muscle) when switched over. So perhaps it's another option to consider.