Probably a mixture of the clay retaining water/ not draining and the shallow bed. I used cavity/ cinder blocks to build up the bed, much cheaper than railway sleeper style beds. Originally had to hand...
Do potatos prefer full sun? The spot they are in gets maybe 5-6 hrs only. If I replant root crops there again I'll probably dig it up and pitchfork it. Otherwise, ive got some deeper in-ground areas with full sun that are far looser deeper down.