Answer:
not a reason? if you ask a reason ,i can say An actual yield is the mass of a product actually obtained from the reaction. It is usually less than the theoretical yield. The reasons for this include:
-->incomplete reactions, in which some of the reactants do not react to form the product
practical losses during the experiment
->side reactions (unwanted reactions that compete with the desired reaction)
->reversible reactions
->impurities in reactants
but you asked for not reason then it can be anything lul, like balanced chemical equation, rate of reaction and etc etc ✌️;)