Shetland hill sheep tend to move in family groups with each generation learning all the 'gaets' to travel. They move around the hills more than folk think looking for fresh pasture un-grazed by others. Even in the depths of winter they will search out areas of heather and sedge still untouched.
So when it is time to gather them they are reluctant to flock together. They try all the time to sneak off in small groups, which means the dogs have a busy time running around them.