I was walking 3 x week for an hour each time and lost no weight - until I changed what, when and how I ate.

1. You're rewarding yourself in the kitchen. It's easy to see a snack and think, "I can have that; I walked today!" True, you CAN -- but probably not if you want to reach your weight loss goals. It's too easy to eat more than you burned (find out just how much you burned here). For instance, a 30-minute fast walk (15-minute mile pace, or 4 mph) burns 170 calories. You get more calories than that from 2 tablespoons of peanut butter, most full-sized energy bars, or two little ounces of baked potato chips.
Want a "reward" for walking? How about a slimmer waist, a lower risk of heart disease, a clearer head, extra time with your friends, and all the other rewards walking brings? Now that's satisfaction.
2. The dog is slowing you down. How long is your "30 minute" walk with Spot, really? Most dog walkers actually put one foot in front of the other for 8 out of every 60 minutes they're out there with their pups. Doesn't mean you shouldn't walk him, just means you both need to actually walk, or you need to get out there by yourself, too.
3. You don't drink enough. Sometimes, you think your body wants food when it's really asking for water (seriously, sometimes thirst gets mistaken for hunger). You might be sweating out much more than you think. How to know: Weigh yourself before you go and right after you come back. Subtract your finishing weight from your starting weight. Then factor in any water you drank on your walk, because 16 ounces of water weighs 1 pound. So if you started at 150 lb., finished at 149, and drank 8 ounces of water along the way, you need to drink at least 8 ounces as soon as you get back.