Board Thread:General Discussion/@comment-71.237.29.140-20130221003100/@comment-4057437-20130706033114

Sometimes we have returned and the stalhrim has respawned but it still cannot be mined. As with other ores found in the same state, leave the area (a loading screen must occur), and return. It will be able to be mined then.

p.s. Waiting won't work.