When replying on a thread the default editor is a source code view, so when you want to highlight something, instead of seeing it highlighted you will see the source code. You can change this in your personal setting to show the "Enhanced (WYSIWYG) Editor" so that every time you go to post you will see what you type as it will appear on the forum.

Settings (Top right of page), General Settings (left column), (Scroll down to "Miscellaneous Options") Message Editor Interface: Then enable: <input type="radio" name="showvbcode" id="rb_vbcode2" value="2" tabindex="1" checked="checked" style="margin-top: 0px; margin-right: 0px; margin-left: 0px; padding: 0px; width: auto;"> Enhanced Interface - Full WYSIWYG Editing. Now "Save changes" at the bottom of the screen.