Hey ZorbaTHut -
Thank You for the clarification. I understand your issue much more clearly now. Any material can only use a total of 16 Texture Samplers but 3 of those are reserved for lighting information (depending on your shader model you are using). With the picture you have above I am seeing 11 individual samplers which should be within the realm of doable.
What I would do to really test this and see as you go exactly what the errors are, open the Output log in the editor then slowly paint each layer on the landscape and allow each to compile or not. You can see the compile error in detail in the output log (which you can scroll back through as well). Once you get a compile error look at that particular branch of the material and see if anything is amiss in the material setup (most likely a height blend issue, if there is one) In the past, simply painting the landscape with each layer one time allowed the whole shader to compile and removed compilation issues.
Thank You
Eric Ketchum