If all users have closed the PRS and that no other applications are using the shared database then the shared database has one or more orphaned database locks (you will be able to see the offending PC in the PRSMon Computer Name list). Orphaned locks can occur if PC's are shut down abnormally and often follow a database corruption. You can use the PRSMon Tools->Kill Database Locks menu command to forcibly remove orphaned database locks.

See also the Database Maintenance section of the PRS Administration Guide.