I've got a problem with samba and file locking

We have some data files shared with samba, and some windows clients
reading and writing the files at the same time using cobol.

Sometimes (i think that when the client exists abnormally, but I'm
not totally sure), the file is locked by the smbd process.

This is the relevant part of my smb.conf

[global]

oplocks = Yes
Strict Locking = Yes

Any suggestion? I'd like to know other locking configurations with
files being read and write many times from different windows clients.