In case of insufficient intensity of airflow check following

• sense of rotation; remember, that fan with radial rotor, when spinning in wrong direction, pushes out some amount of air from circulation
• spinning speed and tension of belts
• determine intensity of airflow using Pitot head
• determine static pressure of air suction and blow-out. The difference between static pressure of air suction and blow-out determines static pressure of fan.
• check, if tension and pressure answer to values from specification
  

If tension is low and pressure is equal or higher than in specification the causes of unstable work are connected rather with a machine, than with fan.
In this case check losses of tension in circulation, but at the same time control the static pressure of most important parts of machine.

• not correctly adjusted bolts
• two or more curved (blocked) or too narrowed passage between sections
• not enough wide suction grille and air outlet
• blocked filters
• presence of foreign bodies
• turbulence
• the straightforward channel of outgoing air opening is shorter than 250% of channel diameter
  

If intensity and static pressure are low then there is a problem with a fan and accompanying devices
In this case

• check for presence of foreign bodies
• check if some flexible connections are damaged (maybe they are cause of narrowing of air channels)
• check leaking
• check if the fan has a proper suction measuring nozzle
• check for possible obstacles in air sucking (curves, rapid changes is airflow direction)
• check for possible obstacles at air outlet (curves, rapid changes is airflow direction)