I only have experience with my two little girls (now 11 weeks old), but here's what worked for them:
1.
cover the ramp like a tunnel
2.
same fabric: make the surface of the ramp (fleece or whatever) the same as the rest of the cage (so it looks and feels the same)
3. make sure the ramp is absolutely
secure with no jiggling
4.
time and patience: provide the right environment and ongoing love and attention and leave the piggies to discover to explore and do things in their own time and in their own way.
My daughter and I both have felt impatient to introduce new and fun things but the piggies repeatedly show us that they will deal with it in their own ways in their own sweet time. We have to be patient.
Also, rather than trying to coax them, there greatest influence seems to be each other. One tries something new, reports to the other one, and soon they're both up to new monkey business.
I find as my girls feel more and more secure, their curiosity is overruling their skittish natures. The transformation from 6 weeks old (when we adopted to now), to now at 11 weeks, has been remarkable (from totally hiding to now hand feeding).
Perhaps older, calmer piggies just need time to explore new things on their own as well. Some may never like the ramp. Time will tell.