I have seen this sort of problem that is caused by phase coupling and I have also seen the same sort of problem caused by a laptop charger that created interference and made it impossible for x10 signals to be detected by devices plugged into that circuit.
It's fairly easy to sort out the second by unplugging everything that might be remotely suspicious. The first can be more difficult. Fortunately, there is a lot of info here and a lot of people with more experience than I have.
One thing you can do, if you are not sure already, is to determine if both outlets are on different phases. An easy way to do this is to plug an extension cord into one outlet that is long enough to reach the other outlet you want to test.
Drag the cord close to the outlet you want to test. Use a voltmeter or neon indicator light and verify that there is voltage across each plug. Now, take the terminals of the tester and connect them between the narrow flat pin of each plug. If the tester shows voltage (220 volts), then they are on different phases. If the tester shows no voltage, then they are on the same phase. Ditto for other outlets you want to check.