This is an official representative of Viber Media.
The source of this issue could greatly vary. Please read this short page in our HelpDesk, outlining some possible workarounds that will help reboot synchronization of the contact list: http://bit.ly/rRtPqk
(The instructions refer to iPhone, but are relevant to all phones nevertheless).
Let us know if the issue persists.
For more questions or technical support, please visit our official HelpDesk.