Sweet potatoes not sprouting?

I'm trying to sprout sweet potatoes to grow, but nothing is happening? They are local and organic, from the natural food co-op, and I have had them for a month now. I placed them in potting soil and have kept it moist but not wet. I don't have a heating pad but the room I've kept them in is consistently kept above 70°.

Potatoes seem to sprout in days on my counter, but now that I'm trying to grow they do nothing :P What could be happening/should I just give up on them?