Sweet potatoes always take an eternity for us to sprout. I have one in water right now that's been growing roots for 2 weeks and still no sign of any sprouts from the top. It seems like they always know I'm in a hurry and that prompts them to take their sweet time. But once they finally do bud out and start growing slips, there's no stopping them!
If I were you I'd start them indoors right now so you are certain to have slips by early May.
