If I understand you correctly it is essentially a fire tube boiler, so what is the quality of the feed water particularly with regard to minerals, I am thinking here of scale build up on the water side of the tubes that will interfere with heat transfer, monitoring flue gas temperature through the life of the unit would highlight this problem.
I would be doing an acid clean on the water side to remove this scale, I am sure there will be some, no I can't recommend which acid, the unit makers should be able to advise you.
You clean the fire tubes√
The forced air fan is being triggered at correct temperatures and has no obstructions or impaired performance√
You have no extra load on the unit√
You are using the same wood grade and dryness as always√
Then the water side is not receiving the same amount of heat as previous, and performance has declined slowly.
allan