My neighbor Gary had the same issue last spring. He buried his seed potatoes a good eight inches down, convinced deeper meant bigger. The plants came up so weak and sparse, and his harvest in the fall was just a handful of marble-sized potatoes. He learned the hard way that shallow planting is definitely the way to go for a good crop.
Tried that deep planting method one year, convinced I was going to grow potato giants. I basically buried my hopes and dreams along with those seeds. My harvest looked less like food and more like a bag of lumpy golf balls. The plants put so much energy into just reaching the sun they had nothing left for making actual potatoes. I guess I was really just running a very complicated dirt storage service that season.