pgAgent v3.3 job running vacuumdb command fails

We are running PostgreSQL 9.1.9 with pgAgent v3.3 on a Mac OS X server (10.7.4), and using pgAdmin 1.14.3.  I've set up a job to do a batch step where I have used the command line:

vacuumdb -z -a

The job fails, the "Result" shows 133.  Can anyone share any insight into why this fails, and what I am doing wrong?  The exact same job on a Centos server is working perfectly, so I presume it is something about the Mac that I am not understanding...

TIA.