The thing with copper is quite complicated and is not just a deficiency. Both Iron and Molybdenum are involved in the way the body can use copper, so soils which also have these elements in can worsen the problem. Here we have a known lack of copper but we also have a lot of Iron in the ground. Moles turn loose soil up, the sheep ingest this and it prevents copper uptake from the rumen.
A lot of research was done into this with Hebridean Sheep and eventually a copper bolus was designed - Co-Secure - which is a glass which dissolves slowly over about 10 or 11 months, so gives enough Copper directly, without Iron having an effect.
We find we need to use Co-Secure every two or three years although some people use it every year. We are not obsessive about our sheep being jet black. The bolus seems to be a safer way of administering copper than the rather hit and miss way of licks, or feeding cattle coarse mix (which contains copper) or giving injections and so on.
As well as causing colour fading in dark sheep, lack of copper can cause weakening of the wool fibre and swayback. Although sheep can be blood tested for a deficiency, apparently a true picture of the situation can only be obtained from necropsy - which we have never had done.
The added info about the lamb being daughter of a lamb is relevant morri2. The dwarf thing is possible but as others have said it's more likely to be the other factors mentioned. You might find that she catches up after the winter