Since they state that he's useless and that he's doing nothing, they either deny that he's doing anything about immigration, or, more likely, they don't care about immigration as an issue.
Wrong-o, bucko.
We also care about federal corruption and the lack of prosecution of it literally at the same time as we care about immigration.
The quick, universal and uncomplicated fix is that a manager is to delegate to subordinates work he is unable to get deeply involved with and then monitor their efforts.
It appears here that Sessions is locked in his office with one or so issues. Meanwhile, significant portion of his labor force runs up and down the hallway as suits them and not the command structure.