Gotcha, ok. If it's rooting that's a good sign. Store potatoes are usually "young" so they're going to take quite a bit longer to slip than something was over wintered. I'd just be afraid they treated it in some way that will inhibit or at least slow slipping.
Just give it some time. Worst case scenario you've got something for next year if you can keep it alive/growing. You'll just need to keep it in good light and warm.