Copy Skin Weights needs the same joints in source skin and target skin to work.

To get all joints from source skin, select source geo and run this (MEL):

# 1. Select the geo with the skinCluster that you want to get the joints for
# 2. Execute this (Bind this to a key or drag to a MEL shelf button)
$sel = `ls -sl`; 
select -r `skinCluster -q -inf (findRelatedSkinCluster($sel[size($sel)-1]))`;

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