(hmmm 2 weeks old, but i'll throw my $0.02 in anyway.)
The missing channel can be caused by a slightly shorted cable.

ie, A staple piercing but not completely shorting creating a
capacitive filter. Or a single strand from the shielding touching
or nearly touching the center conductor in one of the F connectors.
This is effectively creating a notch filter with a wide 'Q'
RF frequencies act different to shorts and near shorts.

Being ch 6 is on the top of vhf low @ 82-88 mhz, and ch 7 @ 174-180 mhz,
ie: you could have a short at 95mhz with a band width of 22 mhz which
would clobber ch 6, but not ch 5 or ch 7.

If Cogeco still broadcasts FM radio on analog cable, you could
hook up a tuner to the coax at the different points and see if the
lower radio stations are gone too.