Why would it not happen?

Sarah • Can`t wait for baby #1. Can`t wait to have our own little family
Been ttc two months now. Done absolutely everything right. Due on period 25th and BFN on FRER. 18 months ago I got pregnant on second cycle of trying which ended in miscarriage. Wanted to be emotionally ready before we tried again and now I am really worrying why it hasn't happened so quick this time. I really had high expectations this month but it seems like 25th my period will arrive because the FR and clear blue digital all say not pregnant. Gutted.