My very first attempt at an eUpgrade (I usually travel J courtesy of client).

# eUpgrade credits available = plenty!
my status = E (maybe SE in the AC system)
fare = S (more prcisely S7Z3TPB )
flight time = Friday afternoon; its now Monday morning, so on my count 4 days before the flight = (I think) OK booking window for E pax with a S fare
Flight booked through travel agent (not ac.ca)

.. and it tells me "not eligible for upgrade"

Can anyone tell me why this might be?

Only thing I can think of is that the '4 day prior' booking window for E pax is 4 x 24 hour periods before the flight time, rather than 4 calendar days. Is this the case?