We are into guessing mode. My guess is that the key word here is "new". One possibility is that the tubers you bought had not been stored well and weren't very good.

People often don't realise the most important bit of a dahlia tuber is not the thick fat bit, but the bit where the thick fat bits join together.

Another possibility, if you keep on planting them in the same place, is that they have used up all of something they need in the soil, and you need to prepare the soil again.