A lunar eclipse does not happen every month because the orbit of the Moon is tilted 5.145° to Earth’s orbit around the Sun. And thus, the Earth does not always cast its shadow on the Moon during maximum opposition. Instead, a full moon occurs.
Some flat-Earthers take this as ‘evidence’ that a full moon cannot possibly happen because the geometry does not allow for a 100% illuminated Moon. They are wrong. In reality, a full moon is never 100% illuminated.