There was a deal between the Institution that rules Alhambra (Patronato) and the bank called BBVA to sell the tickets (BBVA is the company behind alhambratickets.com), but this deal expired and has not been renewed, so they cannot sell tickets further than July 31st.
From
June 28th on there will be a new Website :
http://www.alhambra-tickets.es (not yet working, AFAIK) and a new phone number: +34 902 888 001.
So, wait for just five days, and you will be able to purchase your tickets.