Have you tried adjusting the temperature? Some colors need more or less temperature (in your case more because you have under-extrusion).
If you also have this with other colors (after you experience under-extrusion) then you might need to clean the nozzle:
I was having some problems / errors, test pieces worked fine, my parts had problems inc warping and poor extruding. I updated Cura and the firmware and I would say check the grub screw on the filament feeders knurled nut, mine was loose. My UM2 is working fine now.
Link to post
Share on other sites