Are you getting any errors in the job log? That would be the first thing to look for.
Some other things to look at:
Make sure the user of the batch job is authorized to the SNDBRKMSG command
Check the ALLOW and MODE attributes of the SNDBRKMSG command (use DSPCMD SNDBRKMSG) to make sure the SNDBRKMSG command is allowed for *BPGM (batch program) and allowed for the environment it is running in.