Sounds like it could be an 'all of the above'
After 4 months of burning, I'd say it's time to do a little maintenance and see what may be causing the issues...how many tons have you used so far?
since it would be the biggest safety issue, i'd start with checking the power vent and horizontal flue pipe for fly ash build up.
then check for ash build up inside the furnace impeding heat transfer.
after that see what your draft is reading to find out if you are sending heat out the flue
then check temps on stove and maybe adjust fan limit