I'm not sure about an X-box in the bottom left-hand corner of the frame but when you see an X-box in the lower right-hand corner of the frame it means that there is text that can't be displayed because the frame isn't big enough (or, looking at it the other way, the text is too big).
To stop this - if this is the case - you need to either make the text frame bigger (not always an option), make the font size smaller (again, not always an option), or link two text frames together so that the text flows from one frame to the next.