What is not a reason why actual yield is less than theoretical yield?

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 ✌️; )