5kSQ Center Diff. Lock doesn't always work...

[Jon Archibald <JArchibald@whpacific.com>]

After reading several helpful (thanks!) responses to my post about the
"Quattro Knob", I decided to
try engaging the 2 "diff. locked" modes while parked with the ignition on.
On the first try, both lights
turned on when the knob was turned to its relative position. I turned the
knob back, and tried again,
only this time, the rear locked, but the center did not. while driving
around town, I tried it again while
sitting at a few rush hour stop lights, and each time the rear lock
indicator came on, but only about half
the time did the center lock indicator light up. Does the car have to be
moving or in a certain state for
the center diff. lock to engage? I apologize if this is covered in the
manual, VAG says mine should be 
to me in 7-10 working days :-). It's just driving me crazy wondering if this
is supposed to be how it 
works or if something's wrong....
Thanks in advance!

Jon
'87 5ksq, SuperBlack/Grey, 5sp