If a Leave request has an incorrect number of hours associated with it, you can go to the leave settings and check the following things:
Leave Autofill settings:
At the bottom of the leave settings is the leave autofill, which dictates how many hours will be given to a leave request.
Check the tags apply to the correct staff
Check the user has hours in the places where the hours are filled from. E.g If Published roster is selected first, check if the user has published scheduled hours on the day of the request.
Default Leave Hours:
If none of the autofill options are available, it will fill the request from the default hours, so check there is a value here.
If these all look correct, you will need to delete the request and put it through again, or edit the hours manually.