What you said
I had similar problems at my first placement - especially the inconsistency. I was told to begin to note visibly any and every behaviour I saw that did not meet my expectations. If anybody misbehaved, I wrote their name and a mark on the board. I set the number of marks for a sanction quite high (five marks) so that they could get used to it. It seemed to work.
My advice would be to implement a new seating plan and to plan lessons so you do not have to circulate as much.
The expert view
So you weren't issued with your visor of a hundred eyes and the headphones of omniscience when you were sent in? Your training provider should be shot.
Of course, these wonderful artefacts don't exist. No teacher can see or understand everything in their class. You shouldn't worry too much about being perfectly fair. You can't get everyone every time. What you can do is to sanction people you see misbehaving and then the problem erodes over time.
A good tip is to make a seating plan so that most of the troublemakers are near you, interspersed with children they don't talk to. Then make your base the front of the class. Try not wandering around for a while. Sit down, take some weight off your feet. But always face the class, and if you have to move around, adopt a crab-like gait so that you can always face potential troublemakers. Sanction the ones you catch. That's all you can do. If they holler "foul" at you, just explain - once - that this is what you'll do because it's the only fair thing. Good luck.
Tom Bennett's latest book, Teacher, is out now, published by Continuum.