We could not even begin to ascertain what might have gone wrong as there could be many causes. Someone that might be able to help is Wendy Hermon at the Swan Lifeline Rescue Facility in Eton, England. We are not sure where you are located in the U.K., but Wendy would know if there are any reports of illnesses, injuries or deaths from predation (human, domestic or wild animals) of swans in your area. This information may be helpful in understanding what might have happened to the swans. We hope that this information is of benefit to you. The Regal Swan