Check SAML account attributes, email address and NETBIOS user name - all this should be configured for user to appear in vrops UI.
Mostly simple comparison between added/non-added user in AD properties will help to see what is missing.
I've also encountered problems importing users from groups. I opted to import the users individually, although I would prefer to use groups. I haven't done much investigation, but in my case it appears to be caused by special characters, in particular a hash (#), in the group's DN. If I use another group with mostly the same users they are imported.