For any dog, but especially a shy dog, physical force is counterproductive. It does not provide any motivation for the dog to comply and may foster aggression or, with a very shy dog, create more serious fear or submission-based behavior.
If you have a shy dog, consider what type of treat or reward you are using. It may not be motivating enough for the dog. You might switch to a different type of food treat or use something else, like a toy, that the dog likes even more. When teaching a new behavior, use the favorite reward, speak in a soft, happy tone of voice, and place your body in a non-threatening posture (such as kneeling). These steps will help to teach your shy dog that working with you is fun and rewarding.