To me, it's not so much the rejection of creep feed that's a problem, but that they also won't eat hay and straw, so will not be developing their rumens.
I would therefore be looking at giving them some really fantastic smelling fine hay, or the sort of forage mixes that you give problem horses. Again those that smell so good you could almost eat it yourself.
Then I would put one or 2 of the healthy cade lambs back in with them and just leave the extra yummy forage down for a while for them all to eat. Leave them without anything else (apart from water) for a whole day and see how they get on. They won't starve to death in that time and it's amazing how a bit of hunger refines the appetite. Hopefully when they see the healthy lambs tucking in they will follow suit.
Also, I would maybe get a mineral bucket for them. One with molasses in, which again should smell attractive. Sheep use smell quite a lot to help them select food.