As a simple example:
Poor conditions, grown under 4 sets of 2 tube fluorescent fixtures @ 20cm :
Same plant under 1000W HID and supplemented with sun:
Do we all see the difference?
That said, there are other factors than light that I found had an impact on tuberous. The density of the soil and what additives were in the soil (perlite = BAD) were two big ones.
This is a tough one to answer. D. gig. can be a pill at times. Part of me wants to say that in all likelihood it is nothing and that the tuber will throw a new stolon for you, either later this season or next season. That said... Sprouts dying can sometimes mean the tuber rotted and the rot just followed the path to the surface.
So, you have two paths you can follow at this juncture: 1) You can dump the pot and see what there is to see, running the risk of further damaging any stolons that are forming now and setting the tuber further back. 2) You can let it be and hope for the best. Generally, I always took the latter route when I had pots that did not produce. I usually kept them a bit on the drier side to avoid any chance of rot from excess moisture but still provided enough moisture for if the plant were still growing underground.
I hope the best for you.
And now... some eye candy