Employee that doesn’t want to adjust to the needs of the job

We have a long term employee here for 3 years now, who is refusing to work after 3:30pm when the job needs her to work to cover someone else that is out on vacation. We have appointed and hired an office manager that has run into it with her over the 2 years that she has been with the company. The manager is telling me that this is happening over and over again, in that the employee is giving the manager attitude when presented with doing something above and beyond her normal working hours, covering for another employee that is out of work due to illness, or vacation, or that interferes with a vacation that was “accidentally” overlapped with another employee. The employee is good at her job while she is here, but doesn’t want to be flexible, and that’s what the job is. The employee would be paid overtime for her time spent at the job, and her schedule would have to shift in order to not incur overtime. How do you feel that this is best addressed?