When a snake has constricted a live prey then refuses to swallow it, especially a young nervous snake, it can often be because it's owner has hovered over the enclosure or keeps running back and forth checking on the feeding (or hops around all excited and gets their friends to come watch - not saying you did but it's been known to happen). After a snake constricts sometimes it will take some time to settle in to eat, this can take quite awhile with some snakes, others just swallow immediately. When they are swallowing a whole prey item they are incredibly vulnerable to attack on themselves, they can't easily flee or fight back with a mouthful of mouse or rat. Their instincts tell them this so if they are startled or bothered during feeding, or feel at all stressed or defensive, they can refuse to complete the feeding.