You want the names to sound like they all came from the same world. Maybe different regions in it, but the same world.
Researching your names to get them all contemporary and from same general location helps. Somewhat.
But the problem arises even there if you don't pick carefully. I once researched a story and named characters such things as Tom and Kilmeny. That's a real if rare name, there, Kilmeny.
But I got complaints from the writers' group about mixing real and invented names.
It can't just be real. It has to look real.