A little bit more information would help-how do you know they have died-where they tubers, cuttings,plant bought in?
Slugs can decimate them. They need a lot of water, should not be a problem this month??
When did you plant them? Are these Dahlia's that haven't come back from last year?
Earwigs and slugs can both be a problem as they both like the succulent leaves.
I've had mixed results with mine this year. One pot of Bishop of Llandaff are growing very well, while another of Waltzing Matilda perished as it was too wet. I looked at the tubers last weekend and they are pretty well rotted I've also grown some from seed - Bishops Children - which flourished until I hardened them off. While half are still flourishing and leafy, the other half got eaten by slugs and are just bare stalks. One can live in hope for a new set of leaves though