It may be a bug in LR, which has had (and still has) problems with newlines in captions. Or it may be on the ZoomBrowserEX side.
I suggest you post one of the problem photos here, and some of us can take a look at it. (Use the camera icon in the menu bar at the top of the reply window.)
Long before I stumbled on LR, I used and still use Cannon's ZoomBrowserEX to do the preliminary cull on pictures. It also allows me to insert comments identifying the picture in the actual picture file itself rather than an outside database. Importing the pictures into LR brings in all the shooting info, but does not consistently retrieve the comments.
Using EXIFutils I have finally tracked down that LR does an excellent job of retrieving and even updating the user comment ONLY when it is a single line. If I have a multiple line comment LR just ignores it.
Is this a design feature or a bug?? I would like LR to capture the comments automatically which it is obviously capable of doing rather than having to cut and paste thousands of comments. Does anyone have a suggestion on how to make this simpler??
Post your comments here which is the place for bugs and feature requests.
I think this is definitely an inconsistent ZoomBrowserEX problem. The file I had been testing was too big (6.5Mb) to attach so I tried to make a smaller one showing the problems. Sometimes the changes I made with ZoomBrowserEX were picked up by LR and sometimes not, sometimes making a change with ZoomBrowserEX somehow prevented LR from picking up the comment at all. The inconsistency drives me nuts.
ALL the comments I have put in with EXIFutils are being picked up by LR including all the return/line feeds. I need to explore further (maybe EXIFTools), but I can probably pull the existing comments and then rewrite them with EXIFutils so LR can get them. I'll loose anything I have added in LR, but I can work around that.
If you have an image whose changes are not being picked up LR, you could make it available via Dropbox, Google Docs, or use TransferBigFiles.com or YouSendIt.com (free, send me a private message on the form to get my email address). One of us experienced at looking at metadata issues might see what the problem is pretty quickly (or not).