Problem with update with select

[Joe Bob]

I am using MySQL 5.0.16 and am having a problem running an update statement. I'd appreciate any advice you may have.

Here's the statement:

UPDATE ent_acct_inst
SET cust_id = (
SELECT c.id
FROM ent_user u, ent_customer c, ent_acct a
WHERE c.admin_guid = u.id
AND a.cust_id = c.id
AND a.id = ent_acct_inst.acct_id)

When I run it the query parses fine, but I get no rows updated. I know the data in those tables is good so an update should be happening.

Another similar statement is updating just fine:

update ent_acct_rule
set created_by_user_name = (
select display_name
from ent_user u, ent_customer c, ent_acct a
where c.admin_id = u.id
and a.cust_id = c.id
and a.id = ent_acct_rule.ent_acct_id)

I'm stumped as to why one statement would work and not the other. I know it may be hard to diagnose without the tables and data, but any clues?