There are a number of reasons why this may be the case:

a) Once a contact is added or edited in your address book it takes some time for it to be processed and added to the private graphs of your connected contacts.

b) Contacts marked as "archived" or "do not share" are never shared

c) If a contact's data includes profanity, it is likely that it will not be possible to share the contact

 

How do I stop a contact from being shared?

To stop a contact from being shared you can mark it as "do not share".

Also note that no personal information about your contacts is ever shared. Specifically, name and contact details are never accessible to anyone else.