We recently received a phone call from a client who had all of the 'Read More' links disappear from their Joomla web site. We carefully checked the articles in question in the administrator interface and could clearly see the 'Read More' link indicator in the desired place in the articles. But they would not display in the front end, no matter what we tried. It turns out the problem and the corresponding solution was so obvious, it fooled us for a few hours. (Yes, hours!)
Our problem was that we were checking the parameters in the individual article interface. What we needed to do was check the default parameters for articles, where the client must have inadvertantly turned the 'Show Read More Links' to 'Hide'. Once we went into the 'Content' menu, 'Article Manager' and 'Parameters' in this interface, the offending item was quickly identified.