I assume you are using multiple blend modes but have not enabled Multiple CanvasRenderers
. Please click Advanced...
in the SkeletonGraphic component in Inspector and enable Multiple CanvasRenderers
and see if it solved the problem.
For your information, the spine-unity runtime should give you a message as shown in the following screenshot if your skeleton matches the error condition:
If you have any troubles with displaying your skeleton in the future, please check if there are any messages in Inspector.