Police departments are like any other organization. When members perform badly, it can be for a variety of reasons, and the mixture of reasons can vary from department to department.
It is certainly possible for ineptitude to be the fault of the department. A department that has inadequate training procedures will surely turn out a number of inept officers. The same is true if the department is lax in its supervisory activities. Poor training and/or poor supervision can lead to ineptitude and are the fault of the department.
However, it is certainly true that even good departments can have bad officers. No process of hiring, training, and supervision is foolproof. A "bad apple" can get in to even the best of departments.
Bad police officers can be attributed to departmental shortcomings, individual shortcomings, or a combination of the two.