if it is scab they will rub themselves raw on fence posts/gates etc (which is also the way scab spreads, in case you have sheep in neighbouring fields).
I have stopped using Spot-on for my goats, as it only deals with one of the two types of lice you can get. Dectomax injection will deal with both scab and lice, however if you have already applied Crovect then I would not od anything else until you can see if it has worked. These things are quite toxic, so I wouldn't overload the sheep.