Sounds to me as if there might be an break character at the end of the paragraph (e.g. frame break or page break). Are the text threads?
let ID show you hidden characters (Text > Show Hidden Characters) and threads (View > Structur > Show/Hide Text Threads). Place your cursor behind the last character in the text frame. Is there a blue sign at the end that looks link a tag that shows to the bottom? Delete it. Now your text should flow.
I hope my guess was right.
Thanks for the suggestion... but that wasn't it. Still don't know the cause... but thought I'd add that this is a very simple document. No layers, no hidden objects, just text and some graphics. This doesn't happen when I'm working on the initial copy of the document. It only seems to happen when I go back to edit text and insert additional text.
We've been able to duplicate the problem, using the same file but on a different computer. I've been out searching other forums but still haven't found anyone reporting the same problem. Since my co-worker is seeing the same thing when she opens the file, I at least know it isn't me! :-)
Meanwhile, I can't edit this document any further... can't wait to tell my boss that I can't make his desired edits!
Sometime it helps to save the document as IDML-File. Open the IDML and try making your edits.
To follow up on this thread, I've isolated the conditios that cause this to happen. At least in this document. Apparently, this happens to the line immediately following a cross reference.
To work around the problem, I deleted the cross reference at the end of the preceding paragraph. Added the new paragraph that spaned two lines in this document and then went back to the preceding paragraph and inserted the cross reference again.
My cross references have a character style applied to them to apply Bold and turn off hypenation. So it looks like the text I was trying to insert was retaining the connection to that style and with hyphenation disabled, it was causing the following text to vanish.