try putting slug'snail bait traps/pellets down. this sounds like something is eating them before they can establish.

I had some marigold last year for the first time and it is the slugs that eat them they love them so much so that if you have a part of the garden you want to keep them away from experts will tell you to plant marigolds somewhere away from that part and thus they eat marigolds and leave your other plants alone. It is usually only the very young marigold they go for so it sounds like you have loads so that it appears that your plant has disappeared . I use slug pellets that are pet and birds safe. Do make sure that you use a safe slug pellets other wise the birds and the hedgehogs eat the slugs and die .