Descending from FL390 to FL210 was fine when I set the V/S to 2500 fpm on the autopilot. However, when I tried to descend to 2000 using that rate, it just seems to stick with 100 fpm. I tried to redo it by hitting the VS/FA button and reset the rate, but it seems to stick with that 100 fpm. I had to push it down using the joystick. What am I doing wrong?