Isolation manifold exstention

[Deep Thought]

Not just one, but several things must come together for "the" technique to work:

Height of Backplate to manifold - or bands on doubles - either one may cause the physical height to be out of reach

Angle of the Center Knob may be canted forward verse straight up

Make sure that you are strategically positioned horizontal or face forward so gravity pulls the valves toward the back of your neck.  I know this sounds elementary, but when you are in the water you are 3 dimensional and you may not realize your true attitude.  I see this all the time with divers who try a valve shutdown: they start to lean forward, then the reach backward to grasp the valve...only to arch their back and do totally the opposite of what they intended.  The best place to practice this is on a platform at Square or Wazee and have a buddy watch you.  I hate the video camera idea, but if you want to see yourself and isolate the problem, then this may help.    If you are not standing on your head, the doubles will slide back down and away.  This is the biggest culprit, attitude in the water.

Try loosening your harness a tiny bit and see if the unit will slide down toward your head (don't get carried away, I said just a bit)

These are just a couple of the common, but not at all the tricks

Practice, practice, practice...