Simple Query in Oracle Linked Table in MS Access causes full table scan.

Question

I am running a very simple query in MS ACCESS to a linked Oracle table as follows:
Select *
From EXPRESS_SERVICE_EVENTS --(the linked table name refers to EXPRESS.SERVICE_EVENTS)
Where performed > MyDate()
or
Select *
From EXPRESS_SERVICE_EVENTS --(the linked table name refers to EXPRESS.SERVICE_EVENTS)
Where performed > [Forms]![MyForm]![Date1]
We have over 50 machines and this query runs fine on over half of these, using an Oracle Index on the "performed" field. Running exactly the same thing on the other machines causes a full table scan, therefore ignoring the Index (all machines access the same Access DB).
Strangely, if we write the query as follows:
Select *
From EXPRESS_SERVICE_EVENTS
Where performed > #09/04/2009 08:00#
it works fast everywhere!
Any help on this 'phenominon' would be appreciated.
Things we've done:
Checked regional settings, ODBC driver settings, MS Access settings (as in Tools->Options), we have the latest XP and Office service packs, and re-linked all Access Tables on both the slow and fast machines independantly).

Answer 1

Primarily, thanks gdarling for your reply. This solved our problem.
Just a small note to those who may be using this thread.
Although this might not be the reason, my PC had Oracle 9iR2 installed with Administratiev Tools, where user machines had the same thing installed but using Runtime Installation. For some reason, my PC did not have 'bind date' etc. as an option in the workarounds, but user machines did have this workaround option. Strangely, although I did not have the option, my (ODBC) query was running as expected, but user queries were not.
When we set the workaround checkbox accordingly, the queries then run as expected (fast).
Once again,
Thanks