Yes Mimidot, it's the lack of shading or shadow round/under the lips so the lips look as though it a 'cutout' stucked on. Also that outline is a bit too dark near where the corner of the mouth is and I need to shade on some of the white/black stripes to bring out the form <<<<paying attention to the entire shape of the zebra's head, as well as the body.
I hope you understand, if the medium was in pencil it would have been easier because a darker line/shade will come forward in perspective. This is where my learnt and tedious lessons of drawing a sphere (head) + cylinder (the body) come in. Here, my medium is in oil and I am using colour to create that perspective, very subtle colours because the subject matter = zebras is limited. It depends on one's style of course, you can just use plain black and white but the finished piece will be very 2Dimensional/flat.
I hope this helps you to understand some of the art techniques. I worked my piece off this image but have changed the background colour to make it 'Arty' for hanging on a wall. The image is quite flat. Lily