The bottom line is your subordinates will get away with what you let them. More importantly, they will mimic what they see you doing. So, if you want them to do it right, you better be doing it right.
If you are having issues, try to leverage your sphere of influence. For example, inspect your subordinates' cars, weapons, and equipment. Periodically check on their work product. Send them to training whether they like it or not. In other words, there are things you can do that are usually found in your policy and procedures that don't need someone's preapproval.
Don't forget that at the other end of the carrot is the stick that holds it. If you must, start taking away their privileges. Taking away a privilege is not considered discipline. I have made many an officer park their assigned vehicle at the office for a week, instead of taking it home, as part of an object lesson.
I have also changed their work hours, changed their zone assignments, and I have even taken away their laptops and made them do everything by hand to make my point. Afterwards, they may still harbor a less than stellar attitude, but they know what behaviors are acceptable and that you are not afraid to handle those that aren't.
The fact that these four truths will follow you throughout your supervisory career doesn't mean you should stop trying. You can still make a difference by having a positive effect on those who want to do better. A former lieutenant of mine used to teach all his sergeants the same way. He would stress that a great part of our job was to get the very best out of our people one day at a time. He balanced that by saying some days would be better than others but we should keep trying anyway. To follow his advice, we started by doing our job first and then worked from there. I recommend you do the same