Simply, goat feed be it nuts or mix, does not have the required amount of minerals. Goats require the same amount of copper, for example, as a cow !!
Feed a good grower pellet/nut allowing approx. 1 - 1.5kg a head until they start to seriously add weight. Cut it back a bit and see if they maintain condition.
Worm them with an Ivermectin based wormer.............don't use Panacur, it is a waste of money as most worms are totally resistant to it.
If you are in a wet area, it might be wise to check for fluke worm..............this has been a much bigger problem this year, no doubt due to the wet winter & spring.
Make sure they have top class hay ad lib.
DONT feed silage. use haylage with extreme caution...........it could contain Listeria bacteria, goats don't generally recover from this disease.