In my experience ducks have been very reluctant and slow to learn to use ramps. The wider the ramp and the less the angle, the more quickly they will learn. Place them one at a time on the ramp, and guide them gently up the ramp. Good luck with their education.
The day after my children and I had spent weeks building a large 14'x8' Run and a 4'x8' House with two ramps I was worried that all our work may be for not because the ducks seemed to have run interest in the ramps. We picked each one of the 9 Ducks up and placed them in the house. The next day I let them out and they came down the ramp without a problem but again after a day showed no interest in the ramps so we did the same thing and again in the morning let them out without any problem. I decided to place no food in there yard but only provide them water and place the food inside there house on the other side of the ramp. I also place some food on and around the ramp to hopefully give them the idea. When my son went out the put the ducks in he found them in the house and for the last 3 or 4 days every night they are in the house.
I am new to this but Ducks seem to play follow the leader and once one does something all the other do the same. So the ticket seems to find a way to get one to try the activity and then the others we follow. I will post some pictures tomorrow but now I am real happy because the ducks are safe in there house and they have the underneath to be out of the sun during the day.