thankyou very much for your answer.

yes, they are captive swans, so the only way to see some cygnets in the future is to wait for a nice and regular spring without any intermittent temperature.

But maybe I can increase some cares to the couple/babies. Helping them in nest construction (they always build it up to a tree stump), making the place more peaceful (the lake is close to our restaurant), taking out all the hindrances: such as the step between ground and nest (about 30 cm) and the step between ground and lake (about 30 cm).
Do you have any other suggestion?

Some people also suggest me to take out every eggs exept one, incubate artificially (I have a good machine that regulate automatically temperature, humidity and moving), and when I stop to move them (between 33rd and 36th day) put them back into the nest.
It lucks a bit complicate but if it works, we are one month ahead in the season, so with better weather condition and more chanches of success..

thank you again. Max