Trouble Shooting The Seatpost is mechanical, this usually makes it quiet easy to fix any problems or issues you might be having.
Seatpost won’t Lock in the UP position OR falls from UP position while riding, but does lock into the down position
If it is not engaging in just the UP position, then lift up the bottom portion of the protective boot and loosen item (#52) ½ turn. Now try the switch and see if it will engage. REPEAT until you feel a small amount of play up and down in the post. (push down on the saddle, and up pull up to feel for play) Once you feel a small amount of play up and down, the problem should be resolved. At this point, loosening the cap more will not do any good. You can also see proper positioning of top cap # 52 in the maintenance.
Make sure the post is not just sticking (not coming all the way up). If it is sticking, see “Post is Sticky” in troubleshooting section. This could happen if you just changed your inside tube.