Read Ignition!. It is, in fact, fairly difficult to find a combination of fuels that ignite exactly in the manner one wants them to, in a reasonable range of temperatures, and can be stored without decomposing or eating the storage container.
Of course, today, in large part thanks to a large number of people who died horrible deaths investigating this during the time period documented in Ignition!, it's not hard at all. But it was sure hard until the 1960s.
