I'm with problems with this. I modify the `tomcat/webapps/share/WEB-INF/classes/alfresco/share-workflow-form-config.xml` and also `./tomcat/shared/classes/alfresco/web-extension/share-config-custom.xml` but... Changes are not made in alfresco. What can be happening?
First of all, you should never modify a file below /webapps/share/ or /webapps/alfresco/ directly. You should primarily create/modify files in the /shared/classes/ directory and/or contribute files via Alfresco extension modules (JAR/AMP).
Simple questions first:
1) Are you sure you performed the right changes for what you wanted to do? (You provided no details about what you want to change, and how you tried to do it, so I must be this generic.)
2) Did you do a restart of the Tomcat instance? (Most XML configuration changes only apply after a restart and since you did not mention how you proceeded to test I cannot assume you did.)
As you can see from my remarks, the more details / information you provide in your initial questions (and responses), the better we may be able to help you...