The crawler duplicates your content so as to allow for an apparently continuous train of images and/or text. If your rollover depends upon the id of an image in the train, you end up with two images with the same id. In most cases this will mess up the rollover code. But there could be some other problem, or this problem plus one or more other problems. In any case, rollovers within the crawler can work if setup in the right manner for the special situation that the crawler represents.
To really know how to do this in your particular case, I would need to see the page:
Please post a link to the page on your site that contains the problematic code so we can check it out.