How to get quicker at troubleshooting?

I’m a commercial service tech, and have been in the trade for about 6 months. (I skipped a lot of the hard stuff, I know.) I’m able to work my way through a lot of service calls, but it takes me far too long, and I recently learned my company isn’t able to bill all of my time because of it. For example, I’ve taken an entire day to find a low voltage short, more than two hours to find out a unit wasn’t running due to blown fuses, etc.

When I get stuck, I’ll often call my boss or a senior tech for help. It helps a lot, but I want to be more independent and not have to call someone a hundred times a day. I know that a lot of knowledge and speed in this trade comes with experience, but what things can I do to be quicker and more efficient with my time on service calls?