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I'm sorry to hear your dog is constipated. I would start treating by adding a small amount of vegetable or mineral oil to the feedings. Just a tsp or less at first, as you don't want to overdo it and cause diarrhea. You can always increase the amount a bit. The oil will lubricate the intestinal lining and push the contents along the tract more quickly than normal.
Once regulated, stop the oil, but you may want to add a higher fiber to the diet, like oat bran, regular canned pumpkin, or cottage cheese when starting to see firmer stools, or you can even compare what you are feeding now with higher fiber diets sold in the store (fiber content listed on packages).
If the problem becomes chronic regardless, there could be an underlying medical reason on why the intestines are not absorbing appropriate amounts of water, and then a vet visit with some blood work may be in order. If you ever see increased urinating along with constipation, that would be a key sign that your dog is getting dehydrated and needs vet assistance.
Best of luck, and let me know if I can help further. Hope your dog is better in no time.