Hi Johnny, not so sure your problem has already been solved or not its been 3 years now :).
Did you check the logs in the batch table, may be there was too much batch history data that needs to be cleaned,
I am also exprerincing the same issue, that On submittion of workflow at time workflow works perfectly fine and all the workflow items assigned to respective person and on the other hand at time, workflow gets stuck in between and no item were assigned to any user, so may be the batch job data is too much and required to be cleaned as all the batch configuration are perfect