There could be a very small flame producing little heat but enough light to fool the photo cell.
I have seen this only a few times in 40 years........Usually caused by water contamination of the Kero
Is the fuel line restricted by a blocked filter, Does the burner oil pump become noisy ( screaming) just before lockout,
You are not giving much information....A video would help!!
The Back gutter looks very suspect !!
Have the tiles slipped onto the sole board.
How high does the back gutter go up the tiles.
There will be Lots of water hitting the back of that stack