Never used that command, but I would say that your job should not be locking the stream file when the command part is run. But I suppose it depends on what your job is doing prior to the command running. Are you able to drill down the IFS at the time of the run using OPSNAV and locate the file. You should be able to click on properties and the USE tab, which should show what is using or locking an IFS object.
Hope this helps