This may sound cheesy collo but the way my FS explained it to me was it was like throwing dice and waiting for a 6 to come up. It probably won't happen first time or even the fifth, or even ever, but the chances are it probably will come up eventually. Just on a side note, he actually made me throw the dang dice and it took me 14 attempts to get a 6. Should have taken that as an omen.

Also your medical team learns about how you respond at each cycle and can suggest new ways to approach it. In my case, I had 1 cancelled cycle, 4 transfers with day 2 embryos, 1 transfer with day 3 embryos and assisted hatching and then a blast transfer, which was my successful cycle. Each cycle helps them help you better, and hopefully increase your chances of falling pg.