You can encounter this issue when PHP 5.2.0 or above is used on your server. This problem occurs because new 'pcre.backtrack_limit' PHP option is available since PHP 5.2.0 and it brings limitation for regular expression. It can be difficult to find the reason for this problem because PHP doesn't raise any error message for it. You can change this PHP option manually or use simple 'Big Article' plugin which you can download here. It only overrides 'pcre.backtrack_limit' settings, but it's enough for making Joomla 1.5.10 work fine with large article.