Check to see if the canvas is tight enough. The most common cause of stretched canvas sag is the canvas not being stretched enough. This can be remedied with a staple gun and some help from a friend.
Use a staple remover to pull the staples from three sides of the canvas.
Have someone hold the stretcher bars as you pull the canvas tightly over the bar that is opposite from the bar that the canvas is still stapled to.
Staple the canvas in the middle to the back of the stretcher bar.
Put a staple half-way between the middle staple and the corner on each side. Then put a staple between the end and the staple you just stapled on each side. Then put a staple between the staple in the middle and the staple closest to it. Stapling this way makes the canvas stay evenly taunt. Keep doing this until the canvas is stapled on one side.
Turn the canvas to the next unstapled side. Have your helper pull on the opposite side and staple it down while you are stapling your side.
Continue stapling as shown in Step 5 until the canvas is done.
Check the size. Larger canvases can need support in the center. You can give a canvas support by adding a crossbar.
Cut a piece of lattice to the width of your painting.
Lay it across the back of the painting.
Staple it to the stretcher bars on each side with at least two staples on each side.