My research tells me that mines respawn 3 days if cleared, 10 days if not cleared. Too many mines in Gloombound so I figure I missed one, so I went and waited 10 days in Whiterun (Outside of the Hold Gloombound is in)... But nothing respawns. The spare ore sitting around does respawn so I come back to nab those... but sometimes I need more than that.

Anyone know of this issue?

If you cleared the mine, the ore veins take 30 days to "refill", not 3. Every time you go back in for the spare ore, you reset that counter.

http://www.uesp.net/wiki/Skyrim:Mining