I have a dry stack also. I cover mine when the engine is not operating.
A lot depends upon:
--engine condition. As engines age they may put out more soot which collects in the stack to blow out at inopportune times.
--how you run the boat. If you run the boat too easily it may not get hot enough to run efficiently by not coming up to full temperature. Cool combustion chambers do not burn fuel well. Also the slower or lower flow exhaust stream will allow more soot to collect inside the stack to later blow out.
Run the engine harder periodically in gear before shutting down. NOT WOT , just somewhat more than your normal cruise rpm.
--sometimes the fuel. I have no choice in my home port so get ~15% biodiesel and it runs cleaner that I can determine. Yes, burn a bit more but I'll take it.
--Coolant temp. Yes , I'm reaching a bit here but I think it goes along with combustion chamber temps if too cool. What is the thermostat rating. I would not worry to much about this untill all else is looked at