WHY'S MY VAILLANT BOILER NOT FIRING UP?

My Vaillant boiler isn't firing up when heating is turned on or when hot water taps are turned on.
All the sequences appear to be taking place.

When I remove the outer casing and then unclip the inner cover the boiler fires up when hot water tap is turned on.

The fan seems to be working fine.

I'm guessing this is some sort of problem related to air flow, or some blockage somewhere, or a faulty switch somewhere.

A Status Mode check comes up with S.33 - Check Air / Flue system.

Somebody please let me know how to progress from here. Thanks.