Anna, did you plant the whole potato? You need to produce the slips first, then plant these at an appropriate distance apart (can't remember what it is offhand) so they have plenty of room to grow and develop tubers. Here's a link to producing slips
http://members.aol.com/SPVine/garden/garden1.htm. It sounds as though you've planted your tubers too deep, but perhaps if you carefully lift them and take the slips off planting them apart they may still produce some tubers, I would imagine the slips are very long if they're growing from a 2ft depth!
As for ground preparation I planted my sweet potato slips in the soil where I had finished digging up my new potatoes so the soil was soft, deep and well manured (I added a bit more manure to boost the soil after the potato crop), they produced well considering our short season.
Good luck and let me know how you get on, my sweet potato tubers are beginning to sprout slips so hopefully I'll have some to plant later this year. :)