Sorry, I missed that most important word - most unusual for me to not spot the word "fleecel! lol
Oliver Henry, of Jamieson & Smith Woolbrokers of Shetland, told me that in his experience (which, as the chief grader and sorter of Shetland fleeces for the organisation which claims to process 80% of Shetland fleeces in Shetland, is vast!), tender fleece often indicates a cobalt deficiency.
As Fleecewife mentions, lack of copper can result in brittle tips and colour fading (different to sun-bleaching) in coloured fleeces. I have seen bands of colour change in the staple of fleeces of sheep not given any copper supplementation (over a period of years), suggesting that the need for copper is higher at some points in the year. I had one grey gulmoget whose fleece had very marked dark and light bands the years she had lambs, and was a much more consistent colour the years she didn't have lambs, so that would suggest to me a higher need for copper (probably it would be the copper affecting fleece colour) when she was growing lambs.
The whole of the UK is deficient in copper, cobalt and selenium.
In the far north of England, most commercial flocks are routinely drenched with cobalt and selenium two or three times a year, and on the moorland farm we gave either copper needles or a chelated copper drench half way through pregnancy to our Mule sheep, to prevent swayback in lambs. The Swaledales never had swayback lambs, whether we coppered them or not, but we'd get the odd swayback lamb in the Mules if we didn't copper them.
Texels naturally cope with low copper, and can be adversely affected by too much copper. North Ronaldsays from North Ronaldsay ditto, and are even more sensitive to copper, although people with flocks which have been on the mainland for generations say that they do not need to be quite so careful.
JG does drenches with and without copper. Because ours haven't had copper supplementation for some years, I got the one with copper this time. When that's used up I will get the one without, and alternate. As I only plan on drenching them once a year (after tupping), I don't anticipate problems, but if we decide to drench more frequently then I think I would probably have both types and only use the copper one once a year at most.
If I have any further qualms about fleece quality or any other issues which could be mineral deficiency related, I would talk to the vet and probably get the sheep blood tested. Our animal health adviser in Northumberland said not to bother getting the soil or grass tested as it would be very deficient in copper, cobalt and selenium, as is the whole of the area - which includes where you are, wildandwoolly. But if you do decide to get some tests, do let us know what you find!