I am trying to create a subquery and use the NOT IN operator in my query. But it hangs for long time and no results.

This is my query:
SELECT qryqtrRga.Date, qryqtrRga.rgacode
FROM qryqtrRga
WHERE qryqtrRga.rgacode NOT IN
(SELECT Query6.qcrgacode FROM Query6
WHERE Query6.qcrgacode = qryqtrRga.rgacode);


Thanks!!