PO printing problem

I have maintained the output condition for message condition type in MN01.I have selected the date/time as 1 - Send with periodically scheduled job and maintained the relevant printer.

When I print mass PO in transaction ME9F, one or two PO's are not getting printed.I checked the spool and the spool exist in the log.

When I reprint the missed PO next time,the PO's are printed sucessfully.Checked the spool and looks ok.Replaced the printer but still the problem persists.This happen very random.

Help me out.