I adjust the density on my grass scene using Distribution parameters, but the items look too sparse, even using high density values. What is the problem?
The density at viewport is not indicative of the rendering result, because the number of items is limited by Display->Viewport->Max Items. You should check the real density rendering the scene.
Alternatively, if you want to adjust the density visually, create a smaller grass region (for example 2 m x 2 m) and test different settings there. Once you get a nice result, apply it to the full area.
In scenes where is needed to scatter huge areas using tiny objects, sometimes is better to create a grass patch and use it as source object. You can see a sample of this technique in the Realistic Grass tutorial:
Also you can download a sample grass scene below