insert failing without raising exception: postgresql, psycopg

Problem:
	Database inserts that worked previously are now failing,
	without raising exceptions!

Request:
	Help!

Production Environment: 
	zope-2.7.4, postgresql-7.4.3p2, psycopg-1.1.15, OpenBSD 3.7.
Duplicated under test environment:
	zope 2.7.4, postgresql 8.0.3, psycopg-1.1.15, OpenBSD 3.8.

Details:

When I call the Z SQL method directly, with test data, the database
insert works as expected.

When I call the same method from a python script, the database insert
fails, without raising an exception.  I can only tell the insert failed
by looking at the database and seeing that the data was not stored.

Though the insert is failing, a sequence column in the target table is
apparently being incremented by the failing insert.  (I deduce this 
from the fact that subsequent Z SQL test inserts show sequence gap
of 12 after each failed insert attempt, while no gap occurs when I
call the Z SQL method repeatedly via Zope's "test" tab.)

I will appreciate any suggestions as to how I might troubleshoot this.

Thanks!

- Jerome

psql output showing "gaps" in the sequence number field, corresponding
to the failed inserts:

water=> select meterserial as mser, readingserial as rser, readdate, reading from meterreads where meterserial = 572 and readdate >= '2005-09-01' order by readdate;
 mser | rser |  readdate  | reading 	[annotation]
------+------+------------+---------
  572 | 4382 | 2005-09-02 |   72100
  572 | 4383 | 2005-09-09 |   72230
  572 | 4384 | 2005-09-16 |   72366
  572 | 4385 | 2005-09-23 |   72398
  572 | 4386 | 2005-09-30 |   72513
  572 | 5681 | 2005-10-01 |   72600
  572 | 5726 | 2005-10-02 |   72602	[test before failed insert]
  572 | 5738 | 2005-10-03 |   72603	[test after failed insert]
  572 | 5739 | 2005-10-04 |   72604	[2nd 'test' in a row]
  572 | 5751 | 2005-10-05 |   72605	[test after failed insert]
  572 | 5752 | 2005-10-06 |   72606	[2nd 'test' in a row]
  572 | 5764 | 2005-10-07 |   72607	[test after failed insert]
  572 | 5776 | 2005-10-08 |   72608	[test after failed insert]
(13 rows)

example Z SQL test output:
insert into MeterReads (MeterSerial, ReadDate, Reading
         )
   values (572,
           '2005-10-08',
           72608
        )