I'm experiencing an issue where several details in a current project get removed from the "Placed Details" section of the Detail Manager when, after having been placed on a sheet, I attempt to edit the detail by using the "Edit Detail" tool and selecting the detail.  When I select the detail, the file opens and all appear to be normal--I can make changes and then "Save Detail."  And when I "Update Detail" the changes I've made appear on the sheet where the detail lies.  But in the Detail Manager, the detail is no longer listed on the sheet on which its placed--it is in the "Unplaced Details" section. 

I first noticed this problem when callouts pointing to one of these details appeared blank (after previously having depicted the detail and sheet number).  I have determined that the moving from "Placed" to "Unplaced" in the Detail Manager occurs when I click the "Edit Detail" button.

Has anybody experienced this before?  How might I best troubleshoot this?

Thanks for any input!