It is going to be hard to tell what you are doing wrong, because the typical workflow is not to use a blueprint, but just import the .raw heightmap information for each layer and connect that correctly using the Landscape Layer Blend node.
An example tutorial of the straight forward approach without the blueprint stage can be found below.
Were you able to get this to work in any engine version or is this your first time attempting this workflow?
Be sure you are using the landscape layer coordinate node within your material so it knows there is a layer to blend and map to the landscape based of the name of the layer.
Let me know if you have additional questions.
Thank you,