Why do they have eyeballs sometimes and sometimes they dont???

2D was in two car accidents which led to dents where his eyes should be.
Noodle's design switches a lot, so it depends whether they want to show her eyes or not.
Murdoc usually always has his eyes showing.
Russel's eyes are white, and they never change.

well, 2-D was in an accident, car crash, i think. not sure about the others.

2-d was like that sense a car accident