Absence of evidence is not evidence of absence. Which is to say that trying to explain why you can’t find your ancestor is akin to collecting water in a sieve. But here are a few possibilities anyway:

At the precise moment the National Library was microfilming the parish register page with your ancestor on it, a tiny wink of light crept through the blackout curtains and fell on just the last few letters of the surname, creating a small overexposure and making the name eternally illegible.

Source & Full Story