Sausages look good and casings look full. It is hard to say the reason why they are tough. Could be just that batch of casings??? Were they all out of the same pack?
Natural are all I use, I rinse and soak mine for about an hour, then stuff and let sit over night. Then smoke them if I am smoking, ice bath, sit over night them package up.
If fresh then after stuffing let rest over night then package up. Never too tough to bite through