The order of the blend.
basically if your upper body slot needs to overwrite a slot for an individual arm, you need to re-apply the slot after the initial 2 slots have been applied.
this way it will always apply and change whatever else is being played.
You need to add another layer blend to catch the correct bone as well.
if you do this with a full body montage you can skip the blend and just put the montage node as the last.