In minecraft 1.7.3 I obtained the flint & iron ingots. Why can't I still make flint & steel?

Are you using the right recipe? While some minecraft recipes have variations, most do not.

The iron ingot must be one square above and to the left of the flint. Any other arrangement is not the 'correct' recipe and won't make anything.

Note that some servers, if you are playing online, ban the creation of Flint and Steel because of an old feature that made them weapons of mass destruction and griefing. The problem has been patched out, but some servers maintain the ban.