Listen - the exact same thing happened on a kid's bike that I volunteered to fix a flat on. It had a pin hole on the outside, a typical flat. I patched it and sent the bike home. A couple days later, I heard that it was flat again. I had to go over there for a birthday party anyway, so I took what I needed to work on it. I found the original fix to be okay, but there was a new 1/4" rip on the inside near the valve. It corresponded to a spoke, but the spoke end was not protruding and I could not detect any sharp edges that could do that, particularly since the rubber rim strip was in-place and in perfect condition. In fact, at the location of the rip, the rim strip is doubled, since this is where it's bonded. I patched the rip and blew up the tire, but it didn't hold. The patch also ripped just like the tube. I was stumped, so I went out and bought a new tube, one of the 'Slime' leak-stoppers (20x2.25). This one held and there has been no more trouble, but I have never seen that happen before, and I'm still stumped.