XP orbs won't go up onto carpet

See the attached image. I'm standing at the same level as the ground on the other side of the carpet. There are XP orbs over there, and they refuse to come up over the carpet. Instead, they very slowly go around it. Any reason they shouldn't come up over carpet?

Distance may be a factor, but if I open that gate you see, I'm going to get attacked by zombie pigmen. Also, if I position myself higher, it doesn't help.