Employee refusing to work on weekend when the role is required to work on some weekends

I have some employees who work on the regular hours during the week (and maybe OT), and some weekends based on the business needs. It is clearly stated on the employee handbook as well as on contract.
Weekend work does not happen regularly and is only based on the business needs. I would say average of once in 1 – 2 months. When employee refuses to work on weekends when asked by their manager, what is the best way to deal with this situation? The manager tried with all the members of the group to see who is available but they all refused to work on a particular weekend despite the fact that request came a few weeks in advance.
This happens often and I would like to know, as an HR, what can I do to help solve this issue.
Thanks in advance for your advise.