Did clay cause this?

[TZ-OH6]

This was my first time growing potatoes and I ended up with a knot of 3-4 potatoes at the base of each plant deep in the soil, which I don't think is supposed to happen. The bed (heavy clay soil) was dug out to 2.5 ft and heavily ammended with wood chip compost. The original top soil (not too heavy but still with a good bit of clay in it) was set aside and used to hill the potatoes. In the end I had about 10 inches of topsoil hilled up on the plants and I thought that would be full of potatoes extending out from the plant, but no, just a tightly packed knot of potatoes pretty much where the seed potato was (no fun digging down 10 inches to get potatoes). It might be that rain caused clay to accumulate/pack down over top of the seed potatoes before hilling started because the tubers seemed to be packed into clay, but expansion against the soil could have caused that. Do potatoes need exceptionally light soil? did I over fertilize? not enough sun (southeast side of house, lots of morning sun and a white brick house). Varieties were Red Pontiac and Katahdin