Have you been using the same polyspan all along? That is, the same brand. I have encountered some polyspan clones. These clones are not of the same quality as the original polyspan. When I started using polyspan, it was the Starline brand.
Most of the clones were poor quality. I have no idea who made them as there was no labeling. The product was wrapped around some cardboard with no manufacturer information. The weave was quite coarse in spots, very uneven, and there was no way to fill it with dope or anything else. And, with such large voids, it was rather fragile, tearing easily.