I made my second floor the kitchen, so all the hay, food and water's up there, but I think I'm going to put the water back on the first floor so that when my second pig goes in (if introductions go well) they can eat and drink without being right up each other's bums.
Anywho, getting back to the point! When I first made the ramp, I made the How-To ramp they show in the gallery with the hidey (free up more space, it looked neat, great idea!), but my pig would never go up. I could put him up, he'd run around, enjoy it, and then eventually come back down, but he never did went back up on his own. I thought perhaps he can't get up, so I switched it to a normal sloped ramp with a brick underneath to make it not so steep. Within minutes of putting it in there, with no encouragements, he was zooming up and down on his own!
So, yes, it could be that they are too timid right now and just need some coaxing, or it perhaps your ramp is too steep for the pigs or they're just not able to climb it. Putting perhaps their hay rack up there (if they know it's there) would give them the desired incentive to venture up there more often, if the problem's just some timid piggies that is.