Filter media clogging: This is the most probable cause. Excessive resistance of the filter media leads to a decrease in air volume.

Duct clogging: Severe dust accumulation occurs inside the ducts, especially at elbows or diameter-changing sections.

System leakage: Air leakage points exist in the ducts or dust collector cabinet, which divert the effective air volume.
Improper opening degree of the fan valve: Check whether the fan inlet regulating valve is in the correct position.