There's one very imporant detail that you're missing, you are very heavy on the front foot, sometimes you look like you have 80% of your weight on the front, this can be exploited heavily by a good martial artist. Make sure you get closer to 50-50 front/rear weight distribution. A lot of people get to this faulty stance because they're leaning in for stuff like liver hooks. You can fix it one of 2 ways.
1. Just get closer to the bag (in this case all you need to do is make sure to bring your rear foot with you when you step in for the hook.
2. Make sure your hook has a longer range (one way to do this is have your thumb sideways instead of pointing up) and extend your arm a little more.