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Go around to all of your affected circuits using the tester to locate any good outlets on the circuit vs bad ones. The reason you want to do this is because allot of outlet terminals will corrode over time and cause you to loose connectivity - just like your experiencing now. And if your outlets are utilizing the "push-ins" Such as the one pictured below - you will definately want to change it.
Please see pictoral diagram of and example of this testing method:
Once you locate a bad outlet next to a good one, you will need to open up both outlets and re-do the wiring like I show here. This method is called “pigtailing” and will help to prevent occurances like this from happening again:
When you do reconnect your devices (outlets and switches ) do not utilize the push ins on the back sides of the devices. Rather use the terminal screw on the side. See pics for examples of this:
Do not use this method:
Use this method: