My office chair wont go up or down - Pride Mobility Products Casual CL30KD Lift Chair Chestnut

Depends on the chair, weight and where you're sitting in it. Some chairs are made for big & tall: Big and Tall Office Chairs Discount Big Tall Chairs For Sale Online

If you weigh a certain amount it will lower the chair much quicker than if you were much lighter. This could be due to muscle mass or you're just an all around big boned person.

Maybe it has something to do with his/her weight. Or have the chair not been locked to stay in one stationary position?

To lower the back, lift it until it stops firmly. Now let it fall all the way down. (It might need a little push to go all the way down.) To raise the back, lift it slowly until it clicks once. Each click is a height setting; it will stay at that height. Repeat until the back is high enough. To go back down, start over.
(This definitely applies to Tempur-Pedic TP4100 Office chair.)

the gas lift is bad