Air is lazy. It leaves by whichever route has least resistance, so an uneven fill quietly starves part of the batch. A corner packed tight by a loader bucket stays cold and wet. A dip in the surface becomes a chimney, and air races out of it without touching the compost beside it.
Fill evenly, keep the surface level, and do not press the mass against the walls. Material wetter than planned will settle and compact on its own, so wet batches need more care with fill height, not more fan speed.