tl;dr: I've lost the ability :(

For whatever background: about ten years ago I lived on my bike, fixed all sorts of stuff, and have patched tubes plenty of times. I'm in a smaller town I can commute by bike again so I got a Park patch kit and, a month or two ago, finally got a flat. I patched it up, gave it the dunk test, and still had bubbles coming out. Sad, but it happens. The tube was old and had already been patched twice so I just chucked it and bought a new tube.

I have a few friends just getting into bikes and one, O., got a flat two weeks ago so I patched her tire and taught her how to do it and it all seemed good. Last week another friend had a flat and I let O. fix it to practice what I showed her and she seemed to do it all right but the patch didn't hold. Then I took a turn and my patch didn't hold either. Then a few day's ago O. told me her tire we'd fixed a few weeks back was actually leaking after all.

All this is to ask: have I totally forgotten how to patch tubes or is the rubber cement that came with the kits bunk? I'm batting 0 for 4 here. Maybe I've forgotten some technique in the last decade but I've never had problems like this in the past.

(Also, I have told all my friends to check their pressure and quit pinch flatting!)