They are gorgeous, Yen. You didn't by any chance get them from Lars, did you?
I have had the worst freaking luck with mismolts in S. pretiosa. I had TWENTY nymphs going well after hatching for a couple days, and now I am down to one. Including, but not limited to, sitting water in the cage. It looks like the more cryptic mantids (those prone to mismolts) dwell, even at the young instars, with limited humidity.