newbie: 93 mercury tracer transmission problem

[wuckywuk]

this is an automatic transmission. when i start off in overdrive or drive the car only shift one time. if i start in low and shift manually to drive it still only shift once. then if i shift on up to overdrive it does not change gears again. if i get up to about sixty mph and down shift all the way to low gear it will shift down two times. what could be my problem. the car does not seem to be going into overdrive.
Thanks in advance