Never had that before but I know it works because I did the trick several times. What comes in mind is that there's something wrong with the button set.
Does the extension have a auto-in button or did you use a autodial with the feature access code. It should be the first.
Also check that there's nothing behind the auto-in or incase you have deluxe, put in the huntgroup number.
Also, do some tests on a real station which is programmed in the exact same way and see what it does.
I know I'm not very helpfull at this moment and I know you have a lot of experience yourself so, just check evething step by step and it will work out fine.
Cheers.