Dear Ken Tong,

the drop down menu for your maintenance access is not gone. It has moved to the lower part of your users access rights view. And changed to selection boxes.

This happens, because your new clerk group is not strictly ordered into the inheritance hierarchy of your other maintenance groups.

So it might be possibile and reasonable for a user to be in more than one maintenance group, which is simply not selectable in a drop down menu.

To fix it you can change the values in the 'Inherited' tab in the groups form view. Create a strict hierachy like: Customer -> User -> Clerk -> Manager (or whatever your old inheritance order was).

Stumbled upon this myself when I started with openerp/odoo.
https://www.odoo.com/forum/help-1/question/how-to-add-a-group-to-project-module-properly-39560

EDIT:

Make sure the active user is in the required group to do the operations. Apart from the Access Rights, one can edit the users permissions via Record Rules. Maybe there are some global record rules active that prevent your clerk from creating new Maintenance Requests. Finally, for the issue in your first comment, it might be possible that the delete button is still visible but will throw an access denied error when trying to use it as the wrong user. Test whether the button really works for Clerks when 'delete' is unchecked.

Best regards.