Hey alx027,
I suggest getting you texture samples plugged into your ‘Layer Blend Node’. Then with the node selected, change the preview weights of your individual layers to be able to see them within the ‘Material Editor’ preview window. Also be sure ‘Live Nodes’, ‘Live Preview’, and ‘Live Update’ are enabled.
Here is an example of a Landscape Material I have set up in my own project. I have highlighted all of the points of interest to get your material to preview correctly within the ‘Material Editor’ itself.
Landscape Material
Let me know if you were able to get your material to show correctly, or if you have any other questions regarding landscape materials.
Cheers,