If the theory says it's possible to juggle five chainsaws but nobody's managed to demonstrate it, and I try it without success, have I proven it impossible? Or have I just proven I'm not good a enough juggler?
Exactly, that's the state of the experiment. There's just so many things that could have gone wrong without us noticing. Even though we trust the instrument is working, we don't know if the material under study is exactly as we expect.