You did not create molybdenum isotopes, rather different molybdenum materials, each with natural isotopic composition and wrong atomic weight. Please read the manual: atomic weight (second field) should better not be filled, material number (fourth field) should not be filled, the isotope mass number (integer) should go in the sixth numerical field (and not in the second), and the material name (matching the ones in the low energy neutron library, namely 92-MO, 94-MO, 95-MO, 96-MO) should go in the alphanumerical field, after the mass number.