The Purina Veterinary diet OM is excellent. It is high in protein and trace nutrients, so you can cut down the total amount very safely. We do find that some dogs, especially retrievers, will not lose weight unless the total amount of food is restricted to below the manufacturer's recommendation.
I would switch to the Purina OM diet, and offer two meals per day, and two good walks per day if possible. You should find some sort of measuring receptacle which when filled to the top, contains the correct amount of food for the whole day. Then you should give just less than half of it for breakfast, just less than half at "teatime" or whenever suits your routine. The rest can be given as little treats dotted through the day at whatever times fits with your normal day. It is nice to give them something when they come back from a walk, or you sit down for a coffee, or whatever it is that you do. The main thing is that the whole of the daily food intake should come from the one measure. If they are losing weight, they will be hungry and anything will seem like a treat, even an extra bit of the normal OM food. Feeding little morsels like this will help to keep the digestion working, which uses up a little extra energy (which is what we want).
Weigh your dogs every week or so. If after a couple of weeks, their weight is not reducing, you need to reduce the daily ration. The correct amount of food to give any individual animal is actually the amount of food which produces the correct bodyweight. There is no better rule than that, so as long as you monitor things carefully, you do not need to worry about them getting less than the amount it says on the bag/tin. They will be fine. (This only applies to bright, active dogs. If there is some other health concern, such as vomiting or lethargy, then a proper veterinary check up is needed.)
Sadly many dogs like this will not lose weight unless they are left feeling hungry. However that is how they are designed to be. If they eat enough to feel full all the time, they will be fat. Whatever the food.