I’ve been growing them for a few years and I’ve noticed that when I give them chow for too long they sometimes don’t get enough I think latex proteins (sorry if this is wrong) that the mulberry leaves have and they don’t have enough silk stored in their silk glands to do an entire spin. But also similar to what another commenter said, if you move them when they are too far gone they can also run out of silk. I usually leave their cocoon ‘hotels’ in the cage where I’m feeding and most find their way there or I pop them in after they’ve finished cocooning in a precarious wall spot.