If all of a sudden your Quicktags buttons no longer insert tags in your WordPress Write section, then the problem could be related to one or more WordPress plugins that you have installed.
I started having problems with the more button, as well as a couple of other quicktags that my other plugins added.
I traced the problem down to the Contact Form plugin, which has an option to enable it’s own quicktag button. If I disable just the quicktag and not the plugin, all my other quicktag buttons now work, including the ones that my other plugins added!
If you are having a similar issue with WordPress, try disabling one plugin at a time to see which one is causing it.