Yup, chickens are like that.
That's why every spring when people post "I am building a coop and don't really have enough room for the chickens I want, but I can just build them a second story and so that will count as twice the square footage for them, right?" I am one of the people who always responds that No, it won't.
An "upstairs" usually gets used a *bit*, especially if there is ongoing social strife in your flock, but it is never going to be used anything LIKE the same way the ground floor is. Chickens are really basically ground-dwelling birds. Even chickens that *aren't* silkies <g>
That said, are you CERTAIN your plank is not too steep, and that they have sufficient horizontal room to feel ok about hopping up and down; and how *many* times have you tried manually placing them up there; and is there sufficient light up there for them to easily see. All of which can contribute to not wanting to go up there.
Good luck, have fun,
Pat