There are diseases that cause loss of condition, but in general they come with other symptoms too, most often runny dirty bottoms.
If they're thin they need something, either cake or minerals or both.
If you want to do analyses to get to the route of a problem, you should do soil, leaf and blood. Otherwise you aren't getting the full picture.
We've kept caking right through from last year; although we have grass the ground has not recovered from the previous 'orrid cold wet 12 months, and the grass has little nutrient value compared to a 'normal' year.
You could give them all a drench of a good chelated mineral drench.