And .....?
1) Some query's are not updateable.
In Access Help look up Queries + Troubleshoot Queries + Select Queries
+ I can't update data from a query
2) Perhaps someone set the Query Recordset Type property to snapshot
(read only).
3) If you are opening the query from an event, perhaps it was set to
read only, i.e.
DoCmd.OpenQuery "QueryName", ,acReadOnly