Do the missing scales look like an injury or like they were never there? Sometimes snakes (most notably Dumerils boas) are born with small patches of scales under the chin that are clear, white, or absent. Nothing to worry about. Another possible cause is that maybe some one tried to remove ticks and in pulling too agressively managed to damage the scales. Normal wear and tear can also sometimes damage scales. Either way, as long as the snake is healthy and the area does not look infected or irritated you should be fine. At this size feed about once a week.