The thing with pinch harmonics is the area with your picking hand that you have to hit is small, and moves as your fretting hand moves up and down the neck. When you're fretting the note move your picking hand toward the bridge or up toward the neck and see if that helps. On my Squire, on the high E string fretted at 12, I'm hitting it between the neck pickup and middle pickup. I'm sure your guitar will be different, but if you find that 'sweet spot' you'll be good.