My suggestion was hypothesis to verify that you don't have specific plugin that would be installed in the master and not in the slave site.

Based on your reply, it seems (not sure) that you don't have any PHP error or they are not reported in an cPanel "error_log" file.

If you are able to login in the master with the www. and not without www this mean that you have manage the master website from the www and therefore that you have decided to only accept the www. as the master website.
The domain is save in the multisites master index and when the sharing is defined with a domain, this force to use this one as the master.
So as you have decided to use the www [master domain] to access and share it, you are now obliged to use the WWW [master] to manage it.
If you had decided to manage the master with the www, you would have both possibiltiies.

The problem is that you are sharing the users with the WWW.master website.
If you had decided to share the users without the www master website, you would be able to login with both the www and without the www.
The reason is the session is defined using the domain used by the master.
So use the most generic master that you want (here probably without www)

Yes! That fixed it, for the most part.

I deleted the slave site and recreated it from stonewallfleet.com and now I can log in with or without the www.

Thank you so much!! I didn't realize that would affect it like that.

My only other issue is that the secondary domain, www.stonewallfleet.nicholasjohn16.com, still doesn't work. Is there anyway that I can still have this domain work for the master site?

Doesn't the component direct domains, not listed to a slave site, to the master site?

If you want to share the users between the several master domain and slave site, this will not work.
The reason is that you can only have one domain value that can share the session.
This is a limitation of the apache server (or other) that accept only one session.cookie_domain value. So you can not share it with different domain.