Messages like Could not save; currently locked by user 'XXX' on machine 'YYY' can occur if a user on another machine is editing the same record.
The problem is that if one user is editing a record, you may find that other records are also locked. This is because the database stores its data in "pages" of data that are 2KB in size, so any record that is locked on the page will automatically lock all the others on the same page.
There is also a possibility there may be an orphaned lock, you can eliminate this possibility by compacting the database.