Squeel Like a Pig
Unwound string pinch harmonics are much less sensitive to where you pinch, in my observation. *On the wound strings it is critical. *Try it at different spots with your picking hand, grabbing the string in different locations to see what comes out. *If I am fretting a low G on the E string, *i find that my best harmonics jump out if I grab it at the theoretical "27th" fret, shifting that location up or down in accordance with where my fret hand is 2 octaves down.
If that doesn't help, just try some more gain!