Your terrain should flow along a chain into the planet input. Starting at the top, everything before the default compute terrain has no actual x,y,z position data, compute terrain adds that, so the next displacement has all the information it needs to push the geometry around. But the next displacement after that in the chain won't know where the actual new positions of each triangle pushed with the previous displacement are. Which often won't matter, because most things after the initial compute terrain should be asthetic texturing, and won't need to know where it actually is on the geometry. But anything that does, you then have to add a compute normal which will then calculate new positions of the previous displacement, for you to exploit in the next.
A long way of saying, compute terrain - displacement - compute normal - displacment (eg lateral) , all via input/outputs.
