Seatpost won’t LOCK in the DOWN position (but does engage the UP position)
If the post was functioning properly, and is engaging properly in the up position. Your protective boot has probably been dislodged from the upper ridge (#58)
It is also possible that your Top Cap (item #52) has become too loose. You can check this by locking the post in the up position and checking for up and down play. Push down on the seat and pull up. If you have a fair amount of play up and down, then tighten down the cap just until there is a slight amount of play. See “Proper positioning of Top Cap #52” in the Maintenance section.
If you had tightened the bottom cap nut (item #68) on the very very bottom of the post, this can cause the post not to engage in the down position.