Hopefully your not overwatering, but if the soil is getting warm enough then it should just be a matter of time. I planted one from the supermarket(taro), and it took quite a while to sprout even though it was on a heat mat.

They need consistent warmth (60-70F soil temp) for about 4-5 weeks to sprout, so either it isn't warm enough, or you overwatered since they do not need water to sprout; wet soil can easily lead to rot in Colocasia tubers.

Carefully dig around and take a peek.