Something seems to be killing my lillies, the second batch?

Please help--we pllanted calla lillies in our front yard and lost all of them to what we thought were snails. We replanted, this time, lillies of the nile, and are losing them too!!! Snail killer didn't work. Our lillies are being "eaten" from the bottom up and when we checked the roots, they had a rotten, foul odor. What could it be???