[Zope] insert but updates don't work

I have this rather curious problem with Zope 2.1.3 on WinNT. I have a zsql query which returns a number of records. I want to do an update on the same table that is being queried for each record returned, but get an authentication challenge that I can not pass. If instead I do, eg, an insert into another table with each value returned, it works fine. Is there something special about sql updates? -- Graham Chiu