My extruder is a printed plastic unit, which feeds the filament through a PTFE spacer to the hot end. PTFE isn't the greatest material out as it does tend to deform slightly when hot and under pressure and I've had to drill out my spacer once or twice to stop jamming - it may be PTFE but if the 'ole up the middle ain't big enough, the filament won't get through no how! Out of interest, have you checked how consistent the diameter of your filament is? I understand that not all PLA is the same - some can vary somewhat in diameter.