Very strange roasting results yesterday and today

a number of roasts almost stalled or wouldn’t climb in temp with profiles that I have used for 100’s of loads, with no issue.
Weather has been on a downward slide with snow coming.
Will go through 2010 and 2011 roast logs to correlate with yesterday’s weather pattern.
Have cleaned the chimney and chaff cyclone before the issues – will re check for blockage.
Have started to dis assemble the exhaust fan and clean it.
Stumped!!!