Hello,
When you say, "Endpoint protection 12.1.2 blocking apps installation in windows 8" do you receive any error message or a pop-up from Symantec Endpoint Protection stating it is blocked?
If yes, could you take a screenshot and post it to us?
Secondly, I would suggest you to check this Article:
How Symantec Endpoint Protection Small Business Edition acts on detections on Windows 8 computers
http://www.symantec.com/docs/HOWTO81461
Hope that helps!!