hello, after having created the scalp of my character the fibermesh in some places there are not. I tried to set “maxfibers” to maximum, “coverage” “by area” but nothing… here an image, thanks for the attention 
Hello @Pianta_Grassa!
This is realistic in the sense that if you look at a person who has very short cropped or shaven hair in which all the strands are straight and perpendicular to the head surface, you will always be able to see the underlying scalp when looking straight down at them, no matter how dense the hairs are. Remember that in most cases, the fiber strands are 2-dimensional by default, with no real thickness.
Topology may also affect how the fibers are being generated. As with most things in Zbrush, applying fibers to a mesh with evenly distributed quads as close to square shaped as possible will produce the best results. Irregular or haphazard topology may make itself apparent in the strand distribution.
The trick is to introduce enough variation to the strands to cause them to catch the light enough to obscure the underlying surface. With longer hair, the strands naturally fold over themselves with styling, and tend to twist more to catch the light. Short hairs might require a bit more thought.
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The two main controls for creating a “thicker” hair are “Max fibers” and “Coverage”. The latter influences the width of the strands. Just manipulating these two controls usually will get you in the ballpark of what you want in terms of thickness.
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Even with very short hairs, you want to introduce a bit of curvature, which will cause them to catch more light than a rigidly straight strand viewed from above. Subtle adjustments to the “Gravity” and “twist” controls may help get a bit more curvature in there.
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Note the “Variation” sliders next to each of the main sliders in the Modifiers section. This is a way to add a bit of randomization to the fibers, so they look more natural and less uniform, and ensure that they all catch the light in slightly different ways.
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Finally, you can add Profile segments to make the strands truly 3-dimensional, though this will come with a performance cost. When you attempt to adjust the Profile slider, Zbrush will advise you to use a render-time BPR setting instead. If performance is a concern, you should follow this advice.
Finally, remember to base your opinion of the look on an actual BPR render, and not just the fast preview strands. You may find the rendered fibers to look more realistic than the preview.
Let me know if you have any more specific questions about anything I discussed here, and good luck!