Sorry, I wish I knew. I was just chiming in to say that I've tried Moonflowers for the past five years, from seed, using very reputable seed suppliers, and have not had one Moonflower plant succeed! Either the birds/squirrels get them just as they're starting to sprout, or they don't sprout at all, or who knows what. But I'm not giving up. Next year I'll order seeds again, and try yet again. It's now become an obsession.

Anyway, I hope you figure out what's causing this.

Mine haven't done well in the ground, but this year I planted in 2 containers.
The vines look great and most of my morning glories are blooming just fine, so I'm not sure what the problem is.
Our temps have been triple digits for about 17 days now is my only guess.
As for sprouting, I do a little snip with toenail clippers and soak for about 24 hours.