It may be the Physical position that the Pot is locked down at.
You may have to loosen the connection, and turn the pot in a different spot, then lock it down again.
With some controls, such as Throttles, they are sprung to travel in only one direction. The pot is usually locked down so that it starts out nearly the beginning values (turned all the way to one side). I believe these have to be set up in mame in such a way, to let mame know that its not using a center value start. (single axis, i believe)
With controllers like analog joysticks, the pot tends to start out centered, and so can travel either direction equally. (dual axis, i believe)
You have to be very careful, because if you lock down the pot in the wrong place... and then you use the controller... it could end up moving the pot past the end of its travel stop... effectively breaking it. If it doesnt break completely.. it might just skip to the lowest or highest value.. because it now has looped the thing past where it was supposed to stop.
Also , Some games are designed to accept a certain range of values. If the game sees values that are incorrect, it may cause the kind of things you are experiencing. Either the car/bike going too fast.. or too slow... or completely stopping somewhere in the middle of your throttle.
If the thing calibrates fine in windows... then it could be a mame setting. Delete the INI / cfg files and try again. Also... the particular game itself may need internal calibration. Use the test menu switch, and use the games actual pot test and calibrations. It often tells you what value ranges it should be getting. Or, it will just conform to the new values after you turn the controller to the min and max positions. (but again, only if the pot is set at least in a valid expected range area)