Verify that you correctly spelled the domain that you enter in the slave site definition.
If it is correctly spelled, verify that you have enter the definition with www and without WWW.
When you enter a URL in a browser, it arrives like that in the server and when this is not a "domain redirection", it reaches joomla.
Jms Multi Sites analyze the URL received and verify if it corresponds to one of the domain that you have defined in the list of slave sites.
This is an exact matching. So this is case sensitive.
When there is no matching, it displays the master websites and not the slave.
So verify again the domain name that you have enter in the slave site definition.
If you are using another domain name than your master one, verify that you didn't have redirected the domain to the master. Otherwise, this will be the "master" domain that will be received by the server.
Try review the tutorial video 9 that show a case with a subdirectory. It works on server with Symbolic Link allowed.
Enter exactly the same keyword values.
www.jms2win.com/en/tutorial#jms12x-9
If you are using a cPanel and want to define a subdomain, see the video CP-04.
www.jms2win.com/en/tutorial#jms12x-CP-04