Just a quick post but I have seen that if you applied for Olympic tickets and didn't get them, there are more tickets coming on sale that you can apply for! The equestrian ones go on sale on the 15th of May and there are ones available for all different disciplines.
We have already been lucky enough to get tickets, out of the ones we applied for we got the most expensive ones, therefore if there are different priced tickets and you have enough money I would advise you apply for the more expensive ones. I applied for all cheap ones and didn't get any but my parents got some on their account applying for the better seats.
The link to the Horse & Hound website news story is below. I am unsure of any further details however!
Good luck if you're applying again!