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.