What do you have on the top level? Food or is it more like a lounging area. I found that if I keep the hay upstairs the girls will go up there, they don't go up as much if there is no hay or food up there. Also what kind of ramp do you have and does it have sides. Some pigs are more scared of the ramps than the loft. Also if the ramp is too steep they don't like to go up or down.
I redid my loft today because I didn't like the way it was. It's now a 1x5 L shaped loft. The hay is on one end then they have 3 grids extra space. There is a cozy on the opposite end from the hay box and then I put their little shelving stool up there too. Downstairs they have 1 large and 1 small hooded cozy pillows, their salad bowl pigloo, their bin pigloo, litter box and 2 lengths of pvc piping. Also downstairs is 2 food bowls and 2 water bottles. And they still have lots of room to just run around in.