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Just a guess, but could it be the humidity? Seems like temperature is accounted for, not sure if humidity is dropping too.
What's the strain and how old are the sclerotia? If they're leaking a load of pigment soon after rehydrating, it's the sclerotia that's the problem - comes with an earthy smell. Cold down to 4c won't kill them - I store mine in the fridge occasionally. An hour to two ours is about right for a newly awoken slime to cover an oat flake though. Finely powdered oats can help revive a weak one.
Your best bet is go for broke - take the whole remaining piece, hydrate it and add some powdered oats straight on top - just a pinch. Keep them at 20c min, when you see growth, feed slowly.
It's possible you just got a bad batch of sclerotia, but since both strains are bad, I wonder if they got overheated or something in the post.
I'll post you more on Saturday - they do come with a guarantee!
One Daywalker managed to develop and it seems pretty normal, the second I sprinkled with crushed oats and I think it helped. I'm going to observe them. My other trials proved unsuccessful. I'm going to have a try of this last-chance method today.
Thank you very much for your help both substantive and material.