Complex sentence: Elementary (single-step) reactions do have reaction orders equal to the stoichiometric coefficients for each reactant, but complex (multi-step) reactions may or may not have reaction orders equal to their stoichiometric coefficients.
Split into simpler sentences: Elementary (single-step) reactions do have reaction orders equal to the stoichiometric coefficients for each reactant. Complex (multi-step) reactions may or may not have reaction orders equal to their stoichiometric coefficients.