The last flat I had was caused by a PREVIOUS flat. The punctured casing of the tire slowly wore a tiny, tiny hole in my tube. No problems since putting a patch on the inside of the tire. You can take a cotton ball and "wipe" the inside of the tire. Any tiny protrusion should snag the cotton ball. Do you line up the tire label with the valve? Very handy way to trace the cause of flats...it's helped me a few times, and it's always some barely visible piece of glass or shard of gravel.