that's because they don't digest them at all. In the wild the only way that they eat veges is if its in a animals stomach. So chop it up in a blender really good, and I do mix it with natural yogurt to help digest it at first then slowly ween them off the yogurt if they are doing better. I don't give them any oats or flax seeds but I do give them a bit of oil. Olive oil usually but some times flax oil. Grains are NOT good for dogs at all. At least that is what I have been told which is why kibble is so bad for them. Most dogs are allergic. Stick with the very finely chopped up veges and for fruit I give it to them as a treat because the only benefit is that it tastes yummy to them!
Do you feed them raw meats as well? I do this for my dogs too. They have hardly any digestion problems.
However if they have upset tummies, I give them young coconut water, or pure 100% pumpkin.
P.S. Another thing not good for dogs, Rasins and Grapes are a no no, so are onions. Besides dark chocolate that is all I can think of. Oh and bubble gum but hopefully your not giving your dogs that lol.
Live as if you were to die tomorrow, learn as if you were to live forever ~ Gandhi