This is the result of your where ?
Try
Code:
strWhere = "[fkMemberId] = " & Me!pkMemberId & " and Forms![frmBorrowingBooks].Form.[cntBorrowedBooksSubForm].Form.txtDateReturned is Null"
Debug.Print strWhere
DoCmd.OpenReport "rptMemberBorrowedBooks", view:=acViewPreview, WhereCondition:=strWhere
Do not forget to Dim strWhere
However there is flaw in your logic.?
You are referring to the form control value, so if the first record on the form has no DateRetuened value it works?, if not it states no data, yet subsequent record have no date returned.?
You need to refer to the data in the record not the form controls.