I had a flat in my back wheel because the tire was worn out. I removed it and patched it as in these directions, except for the abrasion with sandpaper, which I never do. I put the tire back in and mounted the wheel, but it deflated again overnight. The leak is under the lip of the patch, which didn't completely seal, as in these pictures:
The hole was around 0.5mm so it is well covered by the rubber cement and the black area of the patch. I have patched many dozens of flats without this problem.
Why does the flat still leak? Is the orange area different from the black area, and does it have to be perfectly glued to the tire?