Yeah this happens to me constantly and it does my head in tbh. You're sat there in the room hearing these clear tones or knocks and then you get home, chuck it on the laptop and half of it just... isn't there. Or it sounds totally different, like the frequency has shifted somehow.
My mate thinks its just the recorder compressing audio weird but I've tried three different recorders now and still get it. The one that gets me is when you catch something on playback that you definitely didn't hear at the time, that feels more significant to me than the other way round.
I've been reading a bit about whether the observer effect plays into EVP work, like whether being present in the room actually changes what gets captured. Probably a stretch but honestly nothing else explains it properly.
Anyone actually tried doing a session where you leave the recorder running in an empty room versus being present? Curious if the results differ. Could be worth testing properly rather than just speculating about it.