Exactly, under-extrusion.
I had some issues with PLA (black) which arrived with the printer, but some time ago I bought two rolls of better quality fillament (red and blue).
I will change fillamant onto red one and print some test cubes again using same settings as i mentioned in my first post. Maybe colour chemical structure is affecting the quality?

But if you have any idea what else I can do to improve prints quality, please let me know Thanks!

IMG_20150823_123320.jpg