This could be caused for a couple reasons. First check to see if you have any up and down play in your post. To check: Push down on the saddle and pull up. If you do not feel any play, go to step 2, otherwise go to step 3.
Lift up the bottom portion of the protective boot and loosen item (#52) ½ turn. 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 play up and down, loosening the cap doesn’t do any good. If this did not resolve the problem, go to step 3.
Check to make sure the switch is working properly. Remove Gravitydropper lid (item #132) Push on switch on handlebar. This should move Magnetic Slider bar (item #140) up and down. You should hear a click when the pin dis-engages. If not look behind the Magnetic Slider bar by pushing switch part way and using your finger to pull front out. The Pin (item 88) could be stuck. Use a large Allen wrench or something steel to attach to the pin and remove. You might need to wiggle the post up and down a bit while doing this. Once the pin is removed, clean and insert back into place. The pin should insert freely. Without installing lid, see if the seatpost is operating correctly. If the pin moves in and out freely but the pin does not disengage when the switch is pressed, your magnetic slider bar might not be moving far enough. Some cable stretch might have developed. See # xxx on shortening the cable in the troubleshooting section.