All you need to do is fill the stitch hole until it is closed. Take the following steps in order to do that:
• Spray a generous amount of impregnation agent onto the seam using a plant sprayer.
• Rub the impregnation agent into the stitches using a brush, paying extra attention to any holes in the seam.
• Allow the canvas to dry.
• Using some water, test whether the seam is watertight again.
• If the seam is still leaking, repeat the treatment until it is watertight.