Sounds like they've rotted................ Are any of them firm.?If so, you need to get the rotted ones off. When you say soft, have they actually gone mucky?

And it sounds like you will be off to the garden centre to choose some new ones.

They do grow beatifully from seed, and mature very quickly, I don't bother with tubers any more. I collect seed heads from my own and other places when out and about during the summer, ready for the following year. So get some planted asap!

On Gardeners' World last night, Monty Don planted a packet of Dahlia seeds - he made the point that they're mixed colours, but he said they make good tubers for you to store at the end of the season.