The first sprouts sometimes take a VERY long time to grow. I often 'knock' them off so the secondary sprouts will take off. They should be fine... just make sure you aren't overwatering. The soil only needs a touch of moisture (cool but not soggy) to trigger the tuber to make roots prior to planting. The sprout will fend for itself or be replaced by the tuber.

I have one like that. It's been out of the ground for more than a month (I planted in the end of April here in NC). It's still alive, and seems ok, but is barely out of the ground. It's really weird, because the one next to it is a good foot tall.

Larger varieties (commonly known as 'dinnerplates') often take a long time to develop the first shoot. The secondary shoots (if the first is cut/broken off) are typically much quicker, but can still take their sweet ol' time getting moving.

It's also possible that the tuber has rotted, and the slow shoot is developing its own roots. If so, it won't grow further until roots are established enough to feed the tuberless plant.

Having patience is very difficult. I can't tell you how many times I have dug up a tuber just to "see" why it wasn't growing, only to knock off the sprout. Sometimes sprouts will temporarily stop growing if they have a shock. If you moved them outside last week they may simply be adjusting to life in a different environment. Also, if it has been cold out at night (as it has been here in MI), they can slow down for a bit until they acclimate or it gets warmer.