This can be a number of reasons: 

1. Most common reason is the email address is incorrect.  Please check the email address is correct inside the contact record. If it is spelt incorrectly this would affect it being pushed up from your CRM account into QuickBooks. 

2. Clear the cache on your browser by pressing both the Control and F5 keys together for PC or the Command + R keys together for a Mac. 

3. Now log back in and try again. 

If you are still encountering problems after checking points 1 and 2 above then please make a note of the contacts not syncing and contact our support desk where we can investigate for you further.