This is one of the bugs I've faced twice when I played with zulu and once when I played with Egyptians. Whenever I save the game (single player/offline vs AI) and load that saved game, I'm unable to spawn any military unit with youth. Nor can I spawn ishikulu from impi, even though I've already researched all the library techs beforehand.
This only happens with African civilisations. Now, what could be the problem? Please provide solution.