There are many reasons that the swans did not return:

Loss of food resources

Better resources on another nearby pond, especially if people are feeding them

Lost on the migratory path or found another site

Inclement weather in your area not conducive to nesting such as flooding of nesting site

Loss of nesting area or too near disturbances such as humans or domestic animals such as dogs, cats, etc.

Presence of predators in the area.

To attract swans, you may need to check the surrounding ponds and lakes to see if there are swans in the area. If so, you might begin feeding the ducks and other waterfowl (cracked corn and poultry layer pellets or a small amount of bread ) (if feeding of waterfowl /wildlife is allowed by your local authorities) on your pond.

Wildlife talk among themselves and if you start feeding, word will get around and if there are swans in the immediate area, they may hear about the feeding, show up and even stay. Good luck. Please let us know if you get swans back on your pond. The Regal Swan