Board Thread:Online Discussion/@comment-69.28.44.220-20150416222937/@comment-6881629-20150417104741

That is mentioned on the quests bugs list here, it takes longer than most mining and you do need a Ancient Nordic Pickaxe to mine any stalhrim (you might need the quest too). Otherwise reload a old save and try again. Leaving the area probably wouldn't help as it doesn't reset.

This is the forun section for the MMO though not Skyrim by the way.