still can't plant dahlia tubers - what am i doing wrong?

Most of the time it's one of three things. Planted too early, so the tubers sit in cold wet soil and rot. Watered straight after planting, same result. Or the tuber has no eye. The eye is on the crown, the bit where the old stem joined, not on the fat part. A loose tuber that broke off without a piece of crown will never sprout however well you treat it. Wait until after your last frost when the soil has warmed up, lay them on their side about 4-6 inches down with the old stem pointing up, and don't water at all until you see shoots, unless it's bone dry.