Prusa i3 MK2 doesn't accept the printing material
My 0.02c:


Cut the tip of the new filament at an angle so that it is pointed enough. This will ensure that it easily enters the hole leading into the hot end beneath the extruder drive gear. A lot of times the curl in the filament (from being wound on a spool) will cause misalignment and lead to it not entering this hole properly.
Check filament debris lodged in the extruder gear teeth. Sometimes this will prevent good grip on the filament.
Finally you can try increasing the nozzle temperature. Often times any residue inside the nozzle will be cleared by elevating temperature and pushing in new filament.