In 7.3 - 7.3.2 inclusive, if you have over 13 locked incidents you may not be able to unlock the 14th and subsequent items - they will appear to unlock, but it will still not be possible to access them.
In this situation the only way to be sure of a successful unlock is to run the following SQL statement:
delete from tps_lock_item
Important: The above statement will remove ALL locks, including valid ones, so is best run when nobody is using the system. Here is a less aggressive statement that only deletes invalid locks that don't have corresponding user sessions:
delete from tps_lock_item where tps_user_session_guid not in (select tps_guid from tps_user_session)
***Always remember to take a database backup before running scripts***
Please note, this issue is not present in 7.4.