Beagle Weight Loss

So last year when we got Darla she was steadily gaining weight. She is a Beagle/Boston Terrier Mix so our originial vet didn't know an ideal weight for her. Well over the winter she blew up to 39lbs even though she was on a scheduled feeding, and organic food, and had 6 walks a day(we didn't have a fenced yard). Since we've moved she is still on a scheduled feeding, on Blue Buffalo-Weight Managment, goes to the park or has a long walk each day, goes to daycare once every 2 wks, and plays with 3 other dogs in a fenced yard. I have tried making homemade treats, and now attempting to use vegetables as treats. I can't just not give them any treats, ecspecially when we are working on commands. My beagle Riley is a good weight 23lbs, but he is on the same routine as Darla to keep him healthy. I'm just getting discouraged, so does anyone have any stories or suggestions? Thanks
Darla & Riley's mom