If you have problems with import/adding contacts, please check the following things:
Does your .CSV file contain a column for email addresses?
If you don't include a column for email addresses (named, e.g., "email address", "business email", "work email", "home email", "alternate email", etc.), then the contacts won't be added.
Does the contact that was not added have an email address?
If you leave the column empty, the contact without an email address won't be added to the contact list.
Did you exceed the allowed number of contacts?
The current number of contacts is limited to 5,000. The purpose of this limit is to prevent slowing of the address auto-complete function. If you had too many contacts in your contact list, it would take longer to scan the list to find the right address.