Hello, what version of Spine is this happening in?
Was there some renaming taking place?
As a workaround, you can try to manually copy the keys, or if that doesn't work, you can try to copy the transforms of the mesh for each key.
Tools - Spine User Guide: Vertex positions
As this can be very time consuming, consider using weighted bones for your animations in future projects. they tend to be lighter, can be easily applied to multiple skins, and only require the initial weight setup.
Blog: Mesh weight workflows
