Well, it looks like I have lost another nymph from my remaining three (I have currently got it on a piece of moist cotton with some food in a little plastic cup on a heat source, but I have not seen any sign of movement since I found it unresponsive this morning, so I think it is dead or nearly so), and another looks pretty weak, and has had some of its antennae clipped (I'm not sure how that happened), so sadly it looks like I may be down to just one out of the four that my dead female originally gave birth to before long.
I'm not sure what could have gone wrong because the middle nymph that now is dying seemed pretty healthy until just a couple of days ago and I had certainly seen it eating since Christmas. The other one was always smaller and weaker and I did think that one might not last long. A couple of days ago I had to hand feed it food and drink which does seem to have perked it up but the antennae have been clipped since then and I am now concerned it might find it difficult to find food at all. Should I isolate it and give it food separately from the adults?
Does anyone have any other suggestions? The adults all look OK and are lively, responsive and seem to be eating well (although no signs of any of the females giving birth yet, even though I have had them over six weeks) and thankfully the one remaining nymph (which was the one that moulted on Christmas day) still looks lively and healthy, but these two smaller ones have gone down hill very quickly, in a matter of a couple of days. I haven't changed anything in relation to my husbandry, the only thing I can think of is that I put some (washed, organic) blueberry and raspberry in the cage for the first time this week and am wondering whether there might have been some pesticide residue still on it that might have caused the sickness with the two smaller nymphs. I am aware that some pesticide use is still allowed in organic farming, although I had hoped there would be less chance of finding residues on organic foods than non-organic, and I don't believe farmers can use systemic insecticides in organic farming so I had thought that just washing everything organic would be good enough. Maybe that's not the case though?
Edited to add: I have now put the small nymph with the clipped antennae in a little plastic cup on its own, with some food and a moistened cotton pad, and I've put the entire cup, with the lid covering it but not firmly clipped on so air can get in, back into the main enclosure nearest the heat pad. I'm going to see if this nymph eats like this, i.e. away from the interference of adults and with food close by, and if so I may keep it like that and if if seems to be getting stronger, and hasn't escaped of its own accord, I'll release it back into the main enclosure with the adults.
The other nymph is definitely dead though

At least I tried I guess but with so few nymphs surviving from my dead female it's still sad to lose another one.