When that happens to me it is because the text is hiding behind another object - like a shaded box - in the content structure. In theory you can fix this using the Order pane, but I find that I get a finer level of control using the Content pane. In either case, scroll through the content to find the object doing the hiding - the shaded box in my example - then drag and drop it before the hidden text in the content order. The hidden text should reappear. Sometimes the object doing the hiding is inside an Artifact container.
Hope this helps.
a 'C' student