Because there is one more condition for becoming part of a coupon exchange that is beyond your control: you have to be part of at least one "order loop". Say you order from user A, who orders from B, who orders from C and so on … then an order loop exists if somebody in that chain orders from you.
If no order loop is found for you, you cannot become part of a coupon exchange. But you could be part of the coupon exchange after that, because people's orders change all the time. Also, if order loops conflict, the PayCoupons algorithm will prioritize yours if you are already waiting longer than others.
Still, there are no guarantees about when exactly your order will come through. We advise you to "save up" coupons of your suppliers because if you need to order them for making an urgent purchase, it may not work in time.