Are you sure the mom does not have them on her back or keeping them under or nearby the reeds? This is normal behavior for swans to hide their young from predators especially at a young age and makes seeing them in a large pond all most impossible. Additionally, she will also keep them between her and the bank even though some of the stronger and older cygnets may stay out in the open and readily seen.

Now, having said this, if none of the above is applicable, then most likely a predator is the cause and the reason swans, ducks, geese even turtles and alligators lay so many eggs because the majority do not survive due to predation. This is nature's way of having a food chain and still continue the species. At a young age, cygnets can be taken by turtles, large fish, mink, bullfrogs, otters, raccoons, egrets, herons, owls, hawks, fox, coyote, alligators and the list goes on. Once the cygnets get to the size of a small duck, the number of predators such as turtle, fish, egrets, etc., decrease. However, throughout their lives, swans are susceptible to large predators including humans and domestic pets. The Regal Swan