The H2n should be picking up stuff that you don't even hear. It's quite common for someone to set up a mic to record, and then after going back, they hear stuff that is normally tuned out. I once did a vocal track upstairs in the bedroom and when I put headphones on and started listening closely, I could hear an airplane fly over the house. I rarely ever hear airplanes unless they are quite noisy. I turn off the fan to my HVAC, don't do laundry and close the door to the cabinet that my computer sits in as the mic will easily pick up that sound. Even my external hard drive sitting on the desk, which REALLY quiet, gets picked up if I turn the mic towards my desk. (Rule #1 is set up the mic to point away from the noise source. ) ECC83 once posted a recording he did in his room and I could hear the clock ticking.
If you can hear voices, cup your ears and spin around until you get the highest level, then point the H2n that direction and turn up the recording gain. If there is sound there, the recorder will pick it up. Once recorded, it's easy enough to bump the gain digitally to make anything stand out.
The only other thing I can think of is if sound is being transmitted though the walls, if you're in an apartment or condo. If you're in a normal house, you shouldn't have any issues with sound conduction through the structure.
