If a request is pending when a new approver is added to the approval flow for a team, the new approver will not see the request, unless they go in the user profile and edit the request*.
Requests are only sent to current approvers.
Please make sure that pending leave is accepted or declined by their previous approvers beforehand. Alternatively please inform all new approvers to check the user requests for their outstanding pending requests.
*Below are steps to edit a request.