[Zope] weird behavior

Perhaps something very stupid I'm doing, but I've a strange result here. I've a ZClass which has the properties 'Title', 'Issue', 'Comment'. This ZClass automatically reindexes itself in catalog. Now, when I change the contents of Issue, Comment and then search the catalog for the specific ZClasses, I get all the created ZClass instances. However, when I change the contents of the Title and I again search the catalog for these ZClasses, I get all but the changed instance back! However, looking inside the catalog, the ZClass is still there! Is this a naming conflict? Or is it something else? Tom.