If possible just attach a screenshot to make it clear. I will try to help before seeing your component properties and codes.
Normally this happens when the width of the StringGrid is less than, or barely equals to, the width of the columns.
So if the bottom slide bar is visible just slide to the left and your hidden column will appear.
If it is the case, fixing this is simple, just give enough room and make the width big enough to contain the columns plus margin for the side slide bar.