Suburban NT-12SE furnace - gas and spark, but no fire - why?

Flame should be high enough to engulf the electrode.......otherwise the milivolt signal can not be generated to 'prove the flame'

If you have cleaned the burner and the main orifice.....then sounds like gas pressure is the problem.

Did you try disconnecting gas line at furnace and blowing it out?

11" WC WITH stove top burners lite, on high and maybe even water heater on
14" is MAX