Append Query (SQL Insert Into) does not insert all values

Hey all,
MS Access 2003.

the following SQL statement runs and works ok except for Field3:

INSERT INTO Table3 ( Field1,Field2,Field3)
SELECT Field1, Field2, Field3
FROM Table2;

it inserts all of the others however Field3 in Table3 remains blank.  The only difference I can see is that Field3 in Table3 has a lookup to another table which has a rowsource of
SELECT Field_ID, Field3 FROM Table1 ORDER BY Field3.

Any ideas why I can't insert into Field3 Table3?