Multiple reasons exist however the most common is the narrow and short muzzle. The construction of their nasal passages also largely contributes to the difficulty of breathing.
Other reasons for snoring are being
-Stenotic Nares are narrowed nostrils. This narrowing causes a restriction in the amount of air that can flow into the nostrils. Surgery is required to enlarged the nares to allow improved airflow. Not all vets have the tools to perform this surgery but once they are found breathing is 100% better for your dog.
One way to determine if nares are an issue is looking up your dog's nose. If it looks pinched or as if you would not be able to put a q-tip up it nares are too compacted and need to be opened
-Elongated soft palate is a condition where the soft palate is too long so that the tip of it protrudes into the airway and interferes with movement of air into the lungs. Surgery is required in many cases where the palate is stretched and the excess tissue is removed. Again not all vets have the tools to perform this surgery but once they are found breathing is 100% better for your dog.
Owners cannot diagnosis this condition as it requires sedation by a vet.
-Overweight: Obesity is an extremely common problem in pets and, as with humans, is detrimental to the health of a dog. Exercise and weight management diet is highly recommended.
-Nasal congestion: Canine sinus congestion could be caused by a number of things, including sinus and viral infections, colds, allergies or even tumors. Main treatment is antibiotics. Consult your vet for necessary tests and treatment.
-Sleep position: If your dog sleeps now with his head flat on the floor try giving him a pillow. If he sleeps on his side adjust him to sleep on his stomach