I've been growing sweet potatoes for 2 years. The yield has been quite small. According to my friend this is what happened to him too but now into year 5 he is getting good production. He thinks the ones bought at the store have been modified to not produce well and it takes a few generations to get rid of that tendency. Sounds plausible.
i havent had to add soil as they grow, just put in good black soil, slightly compacted.
good luck to you, hope you dont have to wait so long. this link may help:
http://www.ehow.com/how_2043375_grow-swe...atoes.html