If it is a DDos attack then their bandwidth would not be able to handle it. Basically it floods the site with requests all the time and keeps customers from accessing the site. The site may seem to work at times because the server happens to be able to handle the load for a short time, but then gets over loaded again. Online bookies make perfect targets because they deal in cash and usually do not have the equipment to handle the attacks. The immediate fix is not really feasible for them because of the volume of clients and that is to block all traffic and manually enter ip address. Of course that only works if you have a static ip.