One probe in the middle tells you about the middle. Build a fixed grid instead: readings near each wall, in the centre, at two depths, taken at the same hours every day.
What you are looking for is spread, not a single value. A wide spread means air is not reaching everywhere, and the fix is usually the fill or a blocked floor slot rather than the fan. Use your nose as a second instrument: sharp ammonia is normal in Phase I, while a sour or rotten egg smell says a pocket has gone anaerobic.