How do I sew canvas and heavy bag fabric?

Canvas is not hard to sew in the middle. It is hard at the corners, where four or six layers meet.

Use a 100/16 or 110/18 needle — a jeans needle works well. A universal 80/12 will deflect and snap.

Lengthen the stitch to 3 to 3.5 mm. Short stitches perforate heavy fabric into a tear line.

Use polyester thread, and consider a topstitch thread with a topstitch needle for visible seams — it reads as deliberate rather than as ordinary sewing on heavy material.

Reduce bulk before you reach the corner. Grade the seam allowances, trim across the corners, and hammer the folds flat with a mallet. This buys you a millimetre or two of clearance, which is often exactly what is needed.

Level the presser foot with a folded scrap as you climb onto a thick junction, so it does not tip.

Slow right down, and turn the handwheel by hand over the thickest points.

The limit is presser foot lift. Below about 12 mm, a machine physically cannot climb a multi-layer bag corner, and no needle or technique compensates. That is where the honest answer is a heavier machine rather than a different method.