pax enter aircraft, walk through isle to their seat and then disappear.
That's not your problem.
Your problem is the PMDG 737 (assuming that's the airplane you are referring to) hasn't been detected, so the Seated Passengers feature can't be enabled, so it's entirely normal they'll disappear, because that's what is supposed to happen if you don't enable them.
It is possible to log the installation this way:
Open a command prompt, which can be done by typing CMD (then Enter) on the Windows address bar
Assuming you installed in the default folder ( if you haven't, you MUST replace this command with your actual path ) type the following:
CD "C:\Program Files (x86)\Addon Manager"
Again, adapt to your actual path if you changed it, and use double quotes if your path contains spaces
Now start the installer this way:
Couatl_Updater2 /INSTALLMODE /LOG
Go in the Config Page, confirm the 737 is not visible, then go back and exit, and check for this file
LOG.TXT
Will if you did everything correctly, would have been created in the Addon Manager folder you were in.
ZIP the LOG.TXT file ( yes, ZIP IT!! ) and attach it to a post or send it by email to support, so I can see what's wrong with it.