The inability to pass stool with ease, should mean an immediate trip to see your veterinarian. Constipation in dogs can be serious and have underlying health concerns as well.
You may find luck with digestive enzymes and probiotics. You can try giving your dog some yogurt, most will eat it without issue.
Consider dehydration if you haven't already. Constipation is often linked to dehydration in both humans and our K9 counterparts. Be sure that your dog has free range access to fresh clean drinking water at all times.
Another alternative to try; canned pumpkin puree for the fiber content. Look to adding a few tablespoons a day to your dogs diet.
The food you are feeding your dog can also be an issue. Some dogs need specific nutrition for their specific breed to ensure they are at peak health.
Keep in mind if you recently switched to a new food this can also cause issues with your dog. Any food changes need to be gradual to avoid problems.