not sure what fixed it in the end, but i made several changes.
I found a Tab in the enterprise console to increase the number of concurrent jobs.
I re- scheduled the job starts so they didnt overlap by setting the child job reloads to occur on completion of the primary job.
Since then ive also upgraded to v9, SR2 removing the beta.
I can now execute jobs in paralell again but as I say im not sure which of the above was the proper fix