A fan is rated at a test condition with nothing in its way. In a real bunker or tunnel the air must pass through a plenum, a slotted floor and one to two metres of damp compost. Each of those adds resistance, and the fan moves less air as resistance rises.
The figure that matters is air passing through the stack. DMR states 10 to 15 cubic metres of air per tonne of wet compost per hour for aerated Phase I. Work out what your batch weighs, multiply, and compare that with what the system is actually delivering before blaming the compost.