Apparently on detached drawings if an item is not visible in the view in which it is ballooned, the balloon will become hidden when the drawing is opened. It can be unhidden via VIEW>HIDE/SHOW ANNOTATIONS, but will be hidden again the next time the drawing is opened
We had several cases of this. 1 was solved by our VAR -
'The reason for the strange BOM quantity is that the part has been set to use a different property. Open up one of the incorrect parts and go to File - Properties.
Make sure you are on the custom tab and you will notice that the BOM Quantity dropdown is set to use the Creation Date custom property. Change this drop down to None and save the part and it should show the correct quantities on your drawing.'
The other was solved by the user, although it is a workaround, not a fix -
'The fix was to show all configurations on the BOM – this seemed to enable the correct display of item numbers and quantities. I then hid the columns in the BOM with the configurations that I didn’t want to show. So far, this seems to have done the trick. This was the only work-around I found that worked successfully.'
I hope some of that helps.