I have a workflow that was developed in 2010 and when I updated it in 2013 I cannot check in the associated xoml file. It is because the Title field in the User Workflow Document content type has been set to required. I have to open Designer, click on “All Files” on navigation, then “right-click” the “workflows” special folder and click properties. Then check the checkbox to allow blank for the Title field. Now you can check in the xoml file. Here is the article that helped me out:
http://sharepoint.stackexchange.com/questions/75978/workflow-files-wont-check-in-cant-undo-checkout