Sounds like the 'flame proving' signal is not returning to circuit board
Spark Electrode provides high voltage spark to ignite the gas and is then used a pathway for 'flame' signal (flame rectification) to circuit board
W/O that signal circuit board shuts down DC to gas valve so raw fuel does not accumulate
Could be as simple as spark gap (too wide/too narrow the milliamp signal gets lost)
Could be circuit board issue
Could be spark electrode wire or wire connection
Could be loose wire connector on circuit board
What Brand/Model is the Furnace???