When a ticket is sync from Service Portal to SM and in SM the ticket is resolved, in Service Portal the users will normally see two buttons: Reject or Accept. In other Service Portal tickets, there will be a close buttons.
There are some cases in which these buttons are not displayed so the users can't perform those actions from Service Portal
This problem may happen because the login name is case sensitive.
Make the user in SM to match the LDAP user. It is Case sensitive.