We used to clip our sheltie in the summer to keep her comfortable, and her dense coat would grow back again by winter. But when she got very old (17), her coat never really grew back after that last clipping. Maybe there just comes a time in an old dog’s life when the hair production shuts down, and since shedding is caused by new hairs pushing out the old ones, I guess that would stall the shedding process, too.