Numbers Cell behavior

Adjacent Numbers cells won't merge if one cell is a header cell and the other is a body cell. By default, it seems, the row of cells at the top is the header row and the column at the left is the header column. I have no idea how to undesignate the column at the left as a header column but I was able to just delete it. Then the first column on the left was able to merge cells with the adjacent column to the right of it. In other cases within the table I had to unmerge some cell groups first before I merged them in a different way.