I think that Sally is right - it is a breed thing but it is also an experience thing too. If sheep haven't seen docks when they were lambs and shown by their dams to eat them, then they are less likely to. Our primitives certainly relish them and seek them out at certain times of day. We had a paddock which for some reason was badly infested with docks and after about three years of grazing plus the occasional topping they seem to be under control
Shetlands in particular seem very fond of big juicy spear thistles - it's terrifying watching them eating them and carefully crunching up the spikes

The Hebs are not eating any of those at the moment, maybe it's something they do later in the year (although OH is clearing them at the mo so hopefully there won't be any later in the year

). Nettles seem to be eaten later too, once the nutrient value of the grass is lower perhaps and the sheep need the minerals which nettles bring up from deeper than grass can?