global.asax issue

I've been stuck on this error for days.
I use global.asax to do url rewrite. which works fine on one server but not
the other. they are both windows server 2003 R2 IIS6.

if I do
http://test.com/photo/123
which would rewrite to http://test.com/photo/default.aspx?photo=123

on the server that doesn't work, it gives me a 404 error

but if I go and create a 123 folder under photo and a blank default.aspx, it
would work and display the correct page.
so this means global.asax is not being triggered

There must be a setting somewhere
Please help

Tem