Take it VERY SLOWLY and put food on the ramp, but only as far as they can reach from the level they are on at first, and then when they are comfy with that, stretching that little bit further.
Alernatively, you can try and turn the ramp into a tunnel by sewing a fleece tunnel yourself or ask threelittlepigs or Sarah from Chucklebunnies on here whether they'll'll sew you one for your measurements that can be velcored onto the ramp.
Many guinea pigs do take time (several days worth even in ramp using company) to come to grips with ramps.