The TOP portion of that post is the part that "slides." Completely disassemble it, remove, clean, lubricate, and reassemble. I'm talking specifically, the piece that sits on top of the solid seatpost.

It actually rotates on top of that post, as the bolt head is mounted in a slot, so it can be moved.

p.s. as a side note, you may have to flip the movable piece 180 degrees to get enuf adjustment to go nose up. Front to back.