a/c won't "pull" charge in, no leaks, good compressor

[Chris Covington]

Today my friend and I replaced my compressor which appeared shot (you
could turn it by hand and no pressure would come out, what a pain that is
on a 20v BTW) with a known good one, and misc. o-rings, etc.
The system has no leaks.  I don't know what the device is called that
we're using to charge the system but it's fairly simple: 2 pressure
gauges, one for the high (goes to 400 psi) and one for the low (goes to
around 100psi), 2 connections to the high and low, and one connection to
the tank of R12.  The line to the low has a small canister which you put
the oil in, which gets sucked in with the pressure when everything is
working fine.

However, when we were attempting to charge it, we couldn't get the
compressor to suck in the charge (not even the oil) even though the clutch
is engaging via the low pressure switch being shorted to get it to pull
in the R12.  We tried charging it through the low-pressure switch
connection and the one right near the compressor to no avail.  The
pressure on the high side and the low side stays at the same pressure as
the tank of R12 (approximately 62 psi, the compressor sometimes would go
to 75psi) so it seems like it isn't being compressed into the system.
There are no leaks; the system held a charge before with the shot
compressor and holds the 62 or so psi right now (nowhere near enough).

What can prevent the charge from being pulled in, besides the compressor
itself (which seems fine)?

Chris
'91 200q20v