GUI: RawImage does not update

Hi, i have a weird GUI Problem.

i'm trying to make a very simple Healthbar. I have a GUI Object with a RawImage and in a script i assign a Texture to it. It looks pretty much like this:

 Texture2D _healthBarTexture = new Texture2D (100,1);
 for (int idxHealth = 0; idxHealth < (int)hitPoints; idxHealth++)
     _healthBarTexture.SetPixel (idxHealth, 1, Color.green);
 _player_HealthBar_Image.texture = _healthBarTexture;

The Problem is, that when i start the code multiple times, with different hitPoint-values, the GUI i see does not change. But when i double click in the inspector at the texture property of the rawImage and change one of the values of the texture (like the filtermode) the GUI gets updated. How do i trigger this update? Or am i doing something completely wrong? Is RawImage the right tool for this?

Thanks in advance chef_seppel