yeah, feedback is tough to predict and get a handle on.
It's so dependent on so many factors that you can't plan too much ahead for it.
I remember years ago in one of my first bands, there were some tunes I used to use controlled feedback on. I had it down to an art in our practice room.. knew exactly which way to point the guitar and what notes to hit.
Then the first time we played live I couldn't get any feedback at all.
I looked like a total idiot waving the guitar in front of my amp and just getting this screaching noise.
After awhile ya learn not to rely on it but to just use your intuition and improvise. If it happens.. great. If not, just shrug your shoulders and wait till next time.