Nether potral disappearing (or incomplete)

I have a nether portal of dimensions 21X21 blocks (largest portal possible). For unknown reasons, the portal seems to switch "off" at random. I keep lighting it and it keeps disappearing after a while.

Curiously, the most recent development is the potral having been switched off for a portion of it, see pictures. I cannot re-light the part of it that has become switched off, if I attempt with a flint&steel, the obsidian block catches fire but no portal blocks appear.