Hi Liang,
Does a communication channel poll files from NFS or FTP? Did you check limitations of file modification check functionality mentioned in a SAP Note 2188990 (like FTP server not reporting file size or usage of file content conversion in PO)?
Did you also have a look into workaround mentioned in a blog When is the File Modification Check supported in Sender File adapter channel? (initial writing to a file that is not polled by PO and subsequent renaming of a completely written file to a name / extension that matches file polling criteria specified in a sender communication channel)?
Regards,
Vadim