I read 5% epidural anesthesias fails to work. what causes them to fail?

Several reasons: Here's a few; 1) Its not in the right spot 2) Its in the right spot but too much to the right or left or it may have wrapped around 3)It only works on certain nerves, nerves around the butt and groin and soles of feet won't generally be accessible hence making someone think it failed 4)You still feel pressure which some people interpret as pain.