Do you have it in full sun??
Has the water been able to drain
right straight thru the pot/container,
really fast??
This is what I do to mine at the end of
the season...
I don't over winter them inside..
I let the first frost kill the tops of the
plant,
and just bring inside in the pot/container and all and just put it in my basement, that the only heat is from our home furnace and 2 hot water tanks..
I don't water them, nothing.. just put them there and leave them..
Then come
spring, I look for sprouts and/or water them starting around mid~March/April..
Then they resprout and we start all over again..
My former neighbor used to take them out of the pot/container/soil/dirt and put them in dampened
peat moss or some woodshavings..
But I never did...
Believe it or not, I also have a plain ole
dahlia variety that does over winter outside,
but it's in a raised bed, on the South side of my house, with a stone wall, and a cement walk way in front of it, and a furnace out vent next to it.
And I mulch it with chopped leaves and keep it
*most of the time* with snow...
And if the January thaws come, I try to shovel
more snow on top of it...