there isn't a photo, check its size and re added it.

Is the one that's struggling ear a window/vent or door so it is getting a chill?
can you see any insects feeding on it?

Hi Seaburngirl photo wont upload. Will keep trying. Both are now on a window ledge. The one that has stopped growing was started on the same window ledge then moved to a greenhouse with no 1 which is going strong. Moved the one that has stopped back to the window ledge about a week ago where the slow starter is happy. Have monitored the problem plant over the last 3 days and there's been no growth whatsoever.

Remove the problem plant from the pot and examine the tuber & roots for damage. Something could be nibbling down there. Give it thorough wash & repot it in a larger pot with new potting mix or plant it in the garden. I'm doing the exact same thing. Some fare better than others & they come 4 in a pack. Don't fuss too much about it. Usually at least one tuber is retarded somehow and I throw it in the garbage and buy new ones.

Thanks Bathgate, I'll give that a go and let you know. It's frustrating that I cant post a photo. I keep getting messages from GOY saying there's a problem.

photos need to be smaller than 3mb but bigger than 3kb.
dont plant out until mid/late May as we will still get frosts in the uK.

Thanks Seaburngirl. Grown Dahlias for many years and never put them in before the last week in May. Have finally succeeded in posting photo thanks to your suggestion.

it could just be down to variety. Did it have few tubers attached to it? If yes then there are fewer sugars for it to get going well.

Hi Bathgate & Seaburn. Well this morning I took the little fella to the greenhouse to check him over and unfortunately found the tuber with the growth to be squishy and those around dead. So it's the end if the road for this particular dahlia. Unfortunately for me I decided that this year I would strip out my border completely and start anew after 30 years! This dahlia was to be in the centre, so now it's back to the drawing board. I did salvage 2 firm tubers and I've planted them to see what happens. However even if they sprout, they're going to be miles behind the other 2. So I'll be at my local garden centre to see what alternative, maturing, dahlia or whatever I can find. The Joy's of gardening!! Thanks for your interest and advice.

You should go ahead and grow the 2 extra tubers. They sprout up pretty quickly and there is still plenty of time for them to catch up with the others. You won't even remember the squishy ones.