Answer
The actual yield of a reaction is often less than the theoretical yield because some of the reactants may not have reacted, and some of the reactants may have formed other products.
Work Step by Step
If some of the reactants didn't react, then this will decrease the amount of product produced because the theoretical yield is calculated assuming all of the reactants react. If some of the reactants may not have reacted, then the reactants will be consumed while not producing any of the intended product. This will give a lower yield because the theoretical yield is calculated assuming all of the reactants react to form a specific product.
