Read carefully all Silverfox wrote about how this can happen with individuals that only appear to be disconnected but have a number.
If a person is truly disconnected and shows by themself in the bottom left corner then go to that person (you can double click and go to a box of info about that person but I prefer to go to their regular page). Note the relationship to you. If ftm says "no relationship" and that is correct then that is why they are isolated. If "no relationship" is not correct then you will have to figure out where they became disconnected and reconnect them via "relationships"