All my hutches are double storey with ramps. I find some pigs take to it like a duck to water whilst others are very reluctant and takes them weeks to get the hang of it. It's usually a more confident piggy here who finally persuades the reluctant user. You cannot force them and setting them on the ramp could just freak them out. Things I have tried are - veg scattered down the ramp, hay trails down the ramp, lifting piggy from the top floor to the bottom floor and leaving for a little while so they realise there is another floor with fun things going on. As others have mentioned you could try sides or a tunnel on the ramp as well.