SharePoint 2007 Designer: Read-only Date/Time disappears with Update Item

Question

I have a simple workflow that creates a User and Date/Time stamp when the user checks complete = Yes. Everything works fine when the columns are unprotected, but when the columns are Hidden or Read-only, the Date/Time from the previous step disappears. I've tried Hiding the fields by managing site content, and by using SharePoint Boost. The workflow goes something like this:

IF Counter = 0 AND Draft Complete = Yes THEN Update Items (Author = Modified by, Draft Complete Date = Modified, Email someone, Counter = 1)

IF Counter = 1 AND QC Complete = Yes THEN Update Item (QC Reviewer = Modified by, QC Date = Modified,Email someone,Counter = 2)

When the fields are hidden, Draft Complete disappears when QC Complete = Yes. I've tried setting Draft Complete Date = Draft Complete Date in the second step, but that doesn't work.

Does anyone know why this would happen and how to fix it? THANKS!