Insert/Delete Trigger Misfires

I am having problems with a trigger that is designed to audit changes to a particular field in a table. If that field is updated, then the old record is inserted into an audit table.

This trigger never fails when I run test data against it from Query Analyzer. It works some of the time when the web application updates it, fails other times.

Typically, multiple records are updated at the same time. Any ideas?

Here is the Trigger:

create trigger t_u_product_rate_detail
on product_rate_detail
for insert, update, delete

as

/--Local variable
declare
@auditdate datetime,
@audituser sysname

--Set values so function isn't executed a bunch of times
select
@auditdate = getdate(),
@audituser = suser_sname()

if exists (select * from inserted)
begin
if exists (select * from deleted)
begin
insert into product_rate_detail_audit_log
select d.product_rate_detail_id,
d.product_rate_id,
d.day_of_week_id,
d.ad_size_id,
d.rate,
d.plan_vol,
d.plan_freq,
@auditdate, @audituser, 'U'
from deleted d
join inserted i on i.product_rate_detail_id = d.product_Rate_detail_id
where (d.rate <> 0 and d.rate is not null)
and i.rate <> d.rate -- this determines if the rate has changed.
end
end


GO
SET QUOTED_IDENTIFIER OFF
GO
SET ANSI_NULLS ON
GO