I would imagine that there is either a problem with your table relationships or the relationship between the tables that you have set up in the query.
From Access Help:
Query based on three or more tables in which there is a many-to-one-to-many relationship.
Updatable: No
Though you can't update the data in the query directly, you can update the data in a form or data access page based on the query if the form's RecordsetType property is set to Dynaset (Inconsistent Updates).
Search for Inconsistent Updates in Access Help.
[This message has been edited by DBL (edited 02-23-2002).]