I am going through the same thing. Planting nice hard tubers with eyes sprouted and they rot. We have had a wonderful Spring with dry weather. Perfect for tubers. I think I have lost 15 or so as some are just sitting and doing nothing.

What is their guarantee? Most places only guarantee their tubers to grow the first season. You might be out of luck if they didn't come back after your overwintering them. It doesn't hurt to give them a call and see what they say. You could also gently dig a couple of them up and see if anything is going on. Maybe they're just late to sprout. I have a few I'm still waiting on.