The time to complete payment is very less and its luck as well as internet speed. Also try to focus on one purchase. I applied 40% successfully most of the times.
I tried that option too. I was targeting 40% discount coupon. But after successfully applying coupon it would give error like people have mentioned above or it would say coupon has expired & try another coupon.
And to do that I had even saved items in cart before.
All this within 5 mins of getting coupon on 4Mb connection.
Coupon quantity is very less IMO. Just imagine 1 lac users trying 1 coupon same time.