The word staff is a collective noun. It represents a group. Some other examples of collective nouns are board, committee, company, organization, department, and faculty.
The trick with these words is that when the group is acting in harmony, you must use the singular form of the verb.
The customer service staff is not available after 6:00 p.m. (This is correct because we are referring to all staff members – no one is available.)
Staff is arguing about the new parking policy. (This is incorrect because the group is not acting as a unit.)??If the members of the collective group are acting separately, you must use the plural form of the verb.
The committee are arguing about the succession plan.
Note: Although this statement is correct grammatically, I believe it is awkward. I would suggest adding the phrase the members of.
The members of the committee are arguing about the succession plan.