deformed chrysalis, and then death

[Pearlandvb]

I have had a whole batch of caterpillars with a problem new to me. Wonder if it's viral or related to a toxin on the leaves?
The caterpillars develop normally, hang upside down as usual, but then the chrysalis can't seem to fully emerge. It comes out half-way, the skin only splits half-way, and then there is a little green chrysalis-like blob in the "head" part. It doesn't advance beyond that, and then after a few days it is clearly dead (body is black, saggy, starts to decompose). No parasites appear to come out of it. When squashed, the chrysalis part looks like the usual egg-yolky material. This has happened to 4 or 5 of the current batch, I've had no normal chrysales. A few have died even before trying to make the chyrsalis, but always at the end stage of caterpillar development.
The plant place where I bought the butterfly weed stated they didn't spray the plant, but I wonder.
Any ideas?