Is the steering on the combine wore out? If the machine always wants to crowd one way when going down the field, you need to change one of the values for the free-play. You can set both left and right value independent of the other.

If you are always off when you turn around on the headlands and return on the last pass, you either have the antenna off center with the header or you have an incorrect offset entered in the lightbar. Change the offset or try moving the antenna to one side by about 6 inches. It should change enough to see which way the antenna needs to be moved