Landscape tool wont let me add some sections to my map, whats the reason for this? (4.18.3)

So I created a map that was too small, I tried to make it larger. However it randomly wont allow me to add some sections to the map, but it will in other places. Anyone know the reason for this, and a way around it?
Screen capture - c4e2a00f67ed737332ac6e5bcd642a4f - Gyazo
^ The last two sections in this video I cant be added for whatever reason.

Odd, but I don’t really think you are supposed to change the map’s core geometry that way.
Export the current heighfiled - if that’s important.
create a new lanscape/layer in the same level that’s larger, import the heightfield.

As the docs say, for performance concerns it’s best to stick to the default map sizes that they suggest.

hmm ok thanks