If you're keying in the correct dates and flights and the required fare is not coming out I would suggest that availability in the bucket you want is not there.

I've just done a test booking for a flight ex-ATH to JFK in November and the low CW return comes out as I would expect. This includes an overnight stop at LHR.

You do need to use the multi-leg option on Expedia, so this is correct.

Remember you need to check availability on EVERY segment of your journey - if one segment is only available in a higher bucket, then the whole journey will be booked in that higher bucket.