[Risolto] Under extrusion - Filament will purge but won't print  

Under extrusion - Filament will purge but won't print

Hi All,

I bought a Prusa Mini about 4 months ago.  I have had mixed results.  It seems that every time I decide to switch the filament, I run into under extrusion problems.  I go through all sorts of attempts to fix the problem (cleaning filament, cold pull, raising nozzle temp, checked heat break to be sure it is clean). 
I am able to load the filament without a problem.  But when it comes to actual printing - there is little to no extrusion.  I can get the filament to purge (i.e. by clicking "change filament" during a print job). 
One curious thing - when I cancel the print, there is a tiny bit of filament extruded at the end.  In other words, once I confirm that I want to cancel the job, the nozzle extrudes a little bit before it parks. 

If the printer can purge the filament, it would seem the extrusion gears are working and the nozzle is not blocked.  So why isn't it extruding when I try to print?  This has happened in the past - whenever I change filament.  I try all sorts of things and eventually it starts working.  Then I am able to print without any major problems - until I change the filament again.  Any suggestions would be great!