Finding records that are not there

Hi:

This is gotta be elementary SQL 101, but I'm having a mental block as
to why this doesn't work.

I have two tables that have identical index fields, maplot and
unitno, (both indexes span two columns) and I want to find all the
records in the commcost table that don't have a corresponding record
in the bldg file.

The SQL I've tried is:

select commcost.maplot, commcost.unitno from commcost
where not exists(select 1 from commcost, bldg
where commcost.maplot = bldg.maplot and
commcost.unitno = bldg.unitno)
order by commcost.maplot

It returns no records although I know that there are records in
commcost which do not match keys with records from bldg.

Help!  What am I doing wrong?

-Owen