Can you check something? Select the Job in Workflow Manager and select the Properties tab at the bottom of the screen. On the Ownership tab, can you tell me if the job is assigned to anyone? If not, try assigning it to yourself and see if that helps. If it is, what about right-clicking on the custom step, selecting properties and seeing who the step is assigned to. If it is unassigned you will want to assign it to your user.
Just some thoughts,