Could be anything!
1 - Steam boilers must not touch - not sure about corners touching but I know side on they cant
2 - you have a valid layout of nine solid or liquid burners underneath ie not mixed or missing? (mentioned above)
3 - you have nub-caked and left a gap in the middle? (I HAVE DONE THIS!)
why did the liquiduct connect? move this
I would suggest building in layers , lay the solid or liquids then 1 layer of 9 boilers and see if they hook up, if not then its probably the touching thing
side note - is your steam consumer pulling in all the steam? On 36 HP boiler you need multiple connections to the consumer to get the 720 steam in