Flat world stopping

I have an Android Samsung Galaxy Tab A. On one of my flat worlds the grass goes to a certain point and then just stops. The whole ground stops. The grass, the dirt, the bedrock. I can place blocks where it has stopped though, and when I fall, I fall a little bit and then stop like there is something beneath me. (There is nothing beneath me though). Please fix this because there is no ground where I would like to build.😞

Thank You😃