Neutrinos are hard to detect because they are 100,000s of times smaller than an electron, so are too small to detect with current equipment. They also dont have a charge and do not interact with any other atomic particles, so it means that it is harder to find when looking at feynman diagrams. The only way we know they exist is because we know that lepton number must be conserved, so there must be an anti lepton emitted during for example beta decay.