With things that get difficult bleeding, what I call sidestepping the pedal can help as it shocks the system and moves the air bubbles.
If you are bleeding alone, and using a valve on a tube to stop air getting back in, bleed as normal, and on the bottom of the pedal stroke, instead of what most folk do which is a steady return, move your foot off the pedal sideways, thus allowing the pedal to return to rest quickly and with a bang.
You'll get 'nil points' for mechanical sympathy, but this technique does work. Taught to me by my father, and most recently used on a year 2000 landrover clutch that was being difficult. Technique also used on brakes for the same reasons.
