Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4

Observation:

Remove and inspect the fan venturis for any signs of damage or restriction / blockage.
You may wish to clean the venturi and recheck operation as even a very fine coating of dust can cause issues.
Note:
Design of venturis may differ slightly and 80 & 90 versions only have one mounted on the flue collector as indicated by the blue arrow.