So I just today finally bought the last missing piece to my frame, the headset.

After putting it on and tightening everything down I took it for a ride.

After I was done I realized that the seat post had moved and gone down, although I had tightened it as hard as I possibly could before breaking anything.

Any ideas what could be happening? I'm using one of the cheaper Leader seatposts.

My thought is that the inside of the seat tube, and the seat post itself, are way to smooth and aren't causing any friction to keep it stable.

Should I grab some light sandpaper and sand the bottom part of the post?