The inconsistency makes it tough. He is aiming for an area of the field where the coverage scheme is designed to focus. If he kicks it somewhere else- shorter, to the other side, even deeper, then coverage can be difficult. Consistency is critical in getting a good net punting average and avoiding big plays against.
I don't think he's having a terrible season. He is in the middle of the pack in net punting average and punts inside the 20. But this is an area he can improve on, IMO.