[Zope] strange zcatalog problem

I have a ZCatalog that seems to lose all it's catalog info whenever I modify almost anything (including things that aren't cataloged), such that my queries that worked before the modification turn up empty after. If I rebuild the indices everything is as normal, but it's really annoying to have to do that after any change I make to anything! Has anyone seen anything like this? (This is Zope 2.3.3.) -- Tim Moore