[Issue] The seatpost is not staying in Descend mode. [Probable Cause(s)] Too much air pressure in the seatpost. If recently serviced, too much internal lubricating oil might be present. [Solution(s)] 
Make sure the pressure in the air chamber is not greater than 25PSI. Make sure the amount of FOX Green 10 wt internal lubricating oil does not exceed 10 mL.