Stumped by new starter

I'm stumped by a problem with a new (rebuilt) Prestolite starter. Bare with
me while I go through the details.

The left engine starter had been causing us problems, the bendix wouldn't
engage every time. We also had a problem with the right bendix but only on
occasion, so we thought we just needed them torn down, cleaned, lubed, and
re-installed. Right starter bendix was cleaned lubed, no problems. Not so
lucky with the left. The shaft was twisted almost 1/4 turn. Currently
bendix parts are back ordered for everybody, so junk it, and install a
rebuilt.

So we installed a rebuilt Prestolite. After we installed it, we cranked the
engine several times and the starter sounded and acted great. This weekend
we flew the Aztec for the first time with the new starter and it acted fine
until we shut down and attempted to re-engage the bendix. This is part of
our normal shut down procedure in cold weather, just to insure that when we
get back in the plane the bendix is already engaged and won't hang up due to
the bendix lube thickening in the cold weather. This time it just sat there
and spun, it wouldn't engage. We tried about 10 times with no luck.

Finally after going into the FBO for awhile, we returned to the airplane and
this time started the right engine first, then attempted to start the left.
Our thinking was that maybe the left engine starter bendix wasn't getting
enough voltage to engage. On the first attempt with the right engine
running, the left bendix spun. The second and third attempt the bendix
engaged and the starter cranked the engine as normal. After we landed back
at home, we left the right engine running, shut down the left, then engaged
the left bendix with the right engine running. It engaged perfectly.

So what's going on? Anybody ever have a limited voltage problem cause a
bendix not to engage? Is this theory plausible? probable?

The battery should have had plenty of juice, this all happened immediately
after a 45 minute flight and both alternators were charging and the battery
showed a normal charge, no discharge.

My next thought would be the starter cable. They are not original cables,
but they are aluminum, not low-loss copper.

Next thing to do I suppose is to pull the cowl and check the
voltage/amps/ohms at the end of the starter cable.

Wondering if anybody has had this problem.

Thanks.

Jim