First check to see if you have any up and down play in your post. To check, Push down on the seat. If you do not feel any play or hear a clicking noise, go to step 2. If you do have play and do hear a clicking noise, go to step 3.
Lift up the bottom portion of the rubber boot and loosen item (#52) ½ turn. Check for play up and down again and REPEAT until you feel just a small amount of play up and down in the post. If this did not resolve the problem, go to step 3.
Check to make sure the switch is working properly. Some cable stretch might have developed not allowing the pin to disengage. Remove the switch lid (item #64) on the handlebar, If you have cable slack in the switch body, loosen the set screws (item #29) with a .050 american allen wrench and pull the excess slack with pliers. Be careful not to pull too much as this will cause the post not to lock into any position.