There are a couple of parameters that limit the maximum number of items (Display rollout) and maximum density (Distribution rollout). If Forest shows a 'F' icon and doesn't display any items, use the "View Stats" windows (General rollout) to check what is the problem and increase these parameters.
However, there is a good trick to create a grassy field without using a huge number of instances:
- Using Forest, create a grass patch with some hundreds of blades
- In Display rollout, Change "Mode" parameter from "Automatic" to "Meshes"
- Create a new Forest object, taking the grass patch as Custom Object. On this, set Mode to "Automatic" (default value)
... and you get a hierarchy of two Forest objects working together. Adjust parameters to get the desired result.
For very dense distributions, we usually use a "Full" distribution map, applying a Random Traslation of 20% to avoid the grid effect.