I’ve had loose canvas after filling on two occasions. Both times the canoe was canvassed during a period of high humidity. Once was during a heavy fog on the shore of Lake Superior and the other time it was under a tarp for a demonstration during a thunderstorm. In both cases I waited for a dry period with low humidity and removed the staples, pulled it tight with pliers, and re-stapled it. From then on it was fine. You know how much your kitchen cabinets swell during humid times.... Well the same happens to canoes, and I suspect cedar swells more than hardwood. I think it important to pick low humidity times for stretching canvas.