Vulcanizing and rubber cement once opened just wont keep in the southern heat without special care. Another problem is you can have several new small tubes unopened but completely dry. I weigh my little Rema tubes and they come in at about 10 grams. If they are less then that they are most likely empty. I also carry a little tube of generic super glue with my patch kit and that has worked for temporary fixes.
I read on this forum of a person who when they get a flat they insert the tip of a tooth pick with super glue on it and after trimming close it has sealed the hole. I have never done this but it seems plausible.
Found a youtube link, but I have never tried it...