You have to use the calculus with each moral action which is largely time consuming, and it is also unclear how you measure pleasure, as you don't know the responses of individuals involved
Why is the calculus a problem with act utilitarianism
Any action should add to the general happiness, and so if you are going to buy someone a gift it would be morally better to give that money to charity instead as it would create more happiness for more people - the happiness of specific individuals don't matter
What is the problem of particular relationships with act utilitiarianism