In addition to the blog post I wrote about CQ5 Client Libraries Not Automatically Refreshing, there is another issue with client libraries that has been introduced in AEM (CQ5.6).
When you create a new client library (cq:ClientLibraryFolder) in AEM, it is not automatically registered in the system as it was in CQ5.5. This means that although your code is there, it cannot be used until Sling recognises it. Adobe are planning a fix for this in a future Service Pack, but until that is released there is a manual workaround.
Navigate to the Html Library Manager in the OSGi console at http://localhost:4502/system/console/components/com.day.cq.widget.impl.HtmlLibraryManagerImpl and stop the component. After the component has been stopped, the page refreshes and the component is not visible anymore – refresh the browser and start the component again. Now your client library will be registered and should be working.
Note that this only applies to newly added client libraries. If you edit an existing client library then you should not need to use this fix.