In our most recent update (V15) the privilege for trashing and archiving orders has been separated. The default is set to 'No'. You have to adjust the privileges for the role in order for your staff users to be able to trash forms (again).
Top menu >> Users
1. Staff roles Tab
2. View the role you would like to adjust/update.
Click Edit in the Privileges box.
Select Yes for the 'Trash forms' privilege.
Click OK when done.