My guess is that you're not fully recording on the tape.
Here's what I think in a bit more detail:
You record a sample on a tape OR You trick yourself about recording on your tape (Rule 101 I think, know your system, then discover the truth, in this example, a protected tape is way much better than toying with tape) Have mercy with your equipment or you might burn your own house down in electrolysis exclamation so badly, you'll have less than a house and wish you never even bought a stereo!
You have two tapes, and press the wrong button to record, you then press another wrong button and play back the tape that you think you've recorded, take it out, rewrap the magnetic tape around another tape in mixing and editing, then obviously nothing is there.
There's also the ability that you are recording correctly, yet where you think you've recorded on the tape is actually much further along the tape strand or more likely, when you rewrap the tape, you're wrapping it the reverse side up, so alike side A, or side B but not quite.
Magnetic interference is highly unlikely unless you're in a thunderstorm area, and all that just is some microscopic odds.
Hope it helped.