Strange that you loose them at that size, Dan. What comes to mind is a nutriant difficiency problem. Make sure the hole you plant them in is well dug over at least a foot around where you are going to plant. Mix compost/rotted manure into that area. Sun flower roots are suprisingly small when you look at the size of the grown stem, so by giving plenty of food in that small area will certainly help. Someone will probably come back and say I'm encouraging leaf rather than flower....it's been said!....Take no notice, put muck in yer hole!
PS. Plant garlic at the base.