Are you using white rice or brown? White would be a more simple carb for them to digest, but even with white, it may go through the system undigested. You can try cooking it longer, as more cooking can break down some of the chains...but this isn't really reliable in terms of predicting whether the dog will be able to break it down.
Dogs digestive systems are more efficient (than ours) at dealing with protein, than with carbs. Also, while humans release enzymes in their mouth as soon as a carb hits their mouth, dogs do not...so the digestion process of carbs, for dogs, is different in many ways.
You could try adding digestive enzymes, but this also isn't a guarantee of digestion. Also, you could try other carbs.
