I figured it out why the shortage of filament extrusion last night finally.
The right amount of force against filament at the geared shaft and idler bearing requires to provide good retraction. Extruding filament after specified length of filament retraction, i.e. 3.5 mm in this case, demands stronger force due to the pressure and friction in Bowden tube.
When the extruder can’t push filament, the geared shaft grinds stuck filament and it ruins entire printing.
In Fisher’s extruder drive, the squeezing force on filament is regulated by spring loaded lever. As I set the bolt length between lever and bolt head to 18 mm, filament starvation began to disappear.
Another important one is that melting temperature of filament affects extruding force.
Lower temperature needs stronger squeezing force at the extruder drive.
Retraction torture test, 205C