Are you tightening the screw only by hand? Is the needle in correctly? Is it pushed all the way up? Assume you already checked all of these, but when frustration kicks in.....
Try this trick. Unscrew the needle clamp almost all the way. Screw it back in. Now loosen it again and insert the needle. Metal burrs can develop on the screw keeping it from going in all the way.
Yes, the vibration could be to blame, on my big machine, the presser foot screw needs to be periodically retightened if the walking foot is on. Or sometimes if I have the pedal to the metal.
And, yes, the needle clamp assembly may need to be replaced; parts have been known to wear out.
If you didn’t hit anything and needle still is straight, you should be able to put it back in. After all, if you forgot to tighten the screw and it fell out, not while sewing, you wouldn’t toss it, would you?
Janomes don’t like strange needles. Mine only likes Organ needles, so I buy them 100 at a time. It might be worth a treasure hunt in your sewing room.