document has tracked changes that aren't marked

[Kevin_at_Midi]

Occasionally, a document that several people have worked on will have tracked
changes that don't appear as such. Using the Next Change and Previous Change
buttons to scan through the document finds the changes, but I'm not able to
accept or reject them, and they're not displayed as tracked changes (no color
or highlight; no outside border line to indicate a change).

I don't know whether this is a bug or a feature that I'm not using properly.
Please help!

Thanks