Dogs need fiber and other nutrients in their diet that can't be gained from liquid food alone. Rather, liquid dog food is either meant as a supplement to speed up weight gain or a topper to make dry food more appealing. If your dog is struggling to eat solid food, your first step should be to see a vet. Then you can try feeding a soft wet food with a pâté-like consistency. You can even attempt to thin this food with liquid dog food.