marigolds shriveling up

I grew a packet of marigold seeds this year, and was delighted to get lots of healthy looking plants.  However, having planted them mostly in containers, I've discovered that quite a few of them are shrivelling up, while the others in the same container are growing well.  Any ideas as to what's causing this.  I dug out the affected plants and there's no sign of vine weavil grubs.