You are aware of your foot positioning, but what about the rest of  your body?
Are you maintaining an organized position?
Torque and Trunk Stability Part 1: How to Stand
Are you doing anything for your feet, ankles, calves or low back?
Some may be building up a tolerance to the amount of time you are on your feet.
Maintaining an organized position throughout your body will help.
Abs on 20% all the time.
Are you doing anything to flush those areas?