I have just started adding grass to my terrain and have encountered this strange problem where I have a gap in the middle of it. When I add more grass to other areas on my terrain the gap gets bigger and I cant add any grass at all to the gap. I hope the picture will explain a bit better.
It probably is working (if you Play and walk around with a guy, you should see it all,) and is just oddness with the Editor.
Try searching “unity grass not showing”. Grass painted only within a circle on the terrain - Questions & Answers - Unity Discussions is a partial answer.
Link to my answer to same Q, on how to “aim” in the editor: Unable to see grass texture - Questions & Answers - Unity Discussions.