I am assuming that you are modifying the Record Detail layout here.
The fields you are referring to are located inside a portal to the Line Items table. It sounds like you have moved the Qty field outside of the portal's boundaries. If you keep the fields the same size and move the Product ID, Description and Taxable fields to the right, then move Qty to the left (you can click an object and then use the arrow keys to move objects one pixel at a time), you should be able to rearrange these fields without messing up the function of this layout. Just note the boundaries of the rectangle that these fields lie within and be sure to keep them inside that border.