If a leave reason has been archived, it won't appear in the leave reason drop-down list when you apply for leave.
To check this, complete the following steps:
Navigate to Admin, Configuration, then Accrual Types.
Select the Leave type that doesn't appear in the drop-down list.
Click Edit and check if the Archive box is selected.
If the archive box has been selected, unselect it and the leave reason will appear in the drop-down list when you apply for leave.