[CentOS] AutoFS mystery ...

I need a second pair of eyes here, please. I have a CentOS 6.6 server
(let's call it 'S1") that has a Samba share on it that is currently
working. We can mount that drive on our Windows work stations and
transfer/delete from it just fine. It's setup as a "guest" config so no
user specific passwords or any other restrictions like that.

I'm trying to setup another server ("S2"), also CentOS 6.6 with autofs to
mount that same share when it needs to. For some reason I can't seem to get
it to work. I get no error message anywhere, no indication of a failure,
nothing.

So on S2 I have:
$ cat /etc/auto.master
/misc           /etc/auto.misc
/mnt/trex       /etc/auto.trex
/net    -hosts

$ cat /etc/auto.trex
trex      -fstype=cifs,rw,noperm,credentials=/etc/cifs-credentials-trex
://bigrip/TREX

The file /etc/cifs-credentials-trex is present with the correct
credentials, and the server also has in its /etc/hosts file an entry that
tells it what IP that 'bigrip' machine is.

Having done all of that and restarting the service (or even the whole
machine) does not auto mount that share and I can't figure out why. I've
completely disabled iptables on both machines, no luck. When I issue an
smbclient command, I get this:

$ smbclient -L bigrip -Uguest
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.6.23-14.el6_6]

        Sharename       Type      Comment
        ---------       ----      -------
        IPC$            IPC       IPC Service (BRASCO)
        TREX            Disk      T-Rex

I should point out that I have a nearly identical setup across two other
machines that is working just fine. I say nearly identical because in that
setup, the first server is actually a Windows Server system while the other
is a CentOS6.6. I've tried as much as I can (or remember) and mirrored the
same setup and config files. Nothing.

So, what am I forgetting here?